"6174"
The food was reheated once, but Xu Tangmian still stayed in the bedroom with her head down.
Yu Chunsheng knocked on the door and came in. Seeing that she didn't even turn her head once and was still writing at her desk, he quietly walked up behind her.
Seeing her gesturing and writing numbers all over a draft paper before crossing them out, Yu Chunsheng couldn't help but ask, "What are you writing? Aren't you going to eat?"
"You scared me to death!" Xu Tangmian clutched her chest and turned around, scolding, "Why do you walk so quietly?"
"I knocked on the door and looked at you for a while," Yu Chunsheng said with a smile, picking up his notebook. "Aren't you a Chinese teacher? When did you start studying math?"
"Didn't I take over the math class of fifth grade three to make a good impression on Director Huang?"
Yu Chunsheng nodded; he remembered that such a thing had happened.
"Later, when I won the Outstanding Teacher Award, the math teacher of Class 1-2 was unhappy. He had been teaching for more than ten years and had been teaching at this school for three years. He felt embarrassed that I had won the honor first. In addition, he heard from somewhere that I had given gifts to the principal, so he challenged me during the day."
Xu Tangmian pointed to the pages of numbers on the draft paper: "Look, I've been studying this. If I can't explain it clearly by tomorrow, I'll have to resign from the school. Then we'll just stay at home staring at each other and staring into the northwest wind."
"So that's how it is." Yu Chunsheng picked up the draft paper, glanced at it, and deliberately provoked her with a smirk: "What's so difficult about all these four-digit subtraction problems? You can't do them?"
“You wouldn’t understand!” Xu Tangmian sighed. “I just don’t get the pattern here. It’s really strange. I can derive arithmetic and geometric sequences, so why can’t I understand this simplest subtraction?”
"It's normal that you don't understand, because this isn't a fifth-grade math problem," Yu Chunsheng said mysteriously.
He snatched the pencil from Xu Tangmian's hand, circled the final result "6174" of each calculation, leaned against the table, and smiled at her. "This number is called the 'Kaprecar constant.' The problem he gave you is actually a conjecture about digital black holes proposed by the Indian mathematician Kaprecar in 1949. You can only encounter it in middle school or higher."
Xu Tangmian was confused and subconsciously retorted, "Are you making up lies to fool me because I can't understand?"
Yu Chunsheng chuckled, put down his draft notebook, and lifted her up by the waist.
Xu Tangmian quickly stood up, her face flushed, and said, "Just tell me to get up, don't touch me."
Yu Chunsheng raised an eyebrow and sat down in the chair, then pulled her over. Xu Tangmian half-heartedly resisted and sat on his lap.
He pulled Xu Tangmian into his arms, turned to a new page of draft paper, and held a pencil as if to write. Before writing, he showed off a bit, saying confidently, "Why would I lie to you? You thought he carefully gave you these numbers to set a trap for you, right?"
Xu Tangmian nodded, "Otherwise, how could it be such a coincidence? I studied it all day, using addition, subtraction, multiplication, division, and square roots, but I still couldn't figure out the pattern."
"It's not that complicated." He casually twirled the pencil, the ordinary pencil spinning between his fingers before returning to its original position, finally dotting the paper and landing on the white paper like a tiny flying insect.
"The point of this question isn't proof, but guiding students to discover a pattern in the numbers." He swung his leg. "What year were you born?"
“1999,” Xu Tangmian answered instinctively.
"Huh?" Yu Chunsheng nudged her again with his leg. "Are you kidding me?"
Xu Tangmian was slightly tilted by his touch, then laughed and hugged his neck, kissing him on the cheek in an attempt to distract him. "Just kidding. I was born, uh... in 1964."
“Okay, now let’s arrange these four digits from largest to smallest and then from smallest to largest.” Yu Chunsheng wrote down the numbers “9641” and “1469” on the paper, “and then subtract them.” He handed the pen to Xu Tangmian.
Xu Tangmian took the pencil and wrote the result after the "=" sign: 8172.
"Then arrange these four numbers from largest to smallest and then from smallest to largest, and subtract them."
Xu Tangmian took the pen again: 8721-1278=7443.
"repeat."
Xu Tangmian paused, turned her head to look at Yu Chunsheng, and looked at him with suspicion.
"Just do as I say." Yu Chunsheng looked up at her slightly and nodded with a smile.
"7443-3447=3996".
"continue."
"9963-3699=6264".
"continue."
"6642-2466=4176".
Without Yu Chunsheng saying anything, Xu Tangmian had already started repeating the process on his own: "7641-1467=6174".
"7641-1467=6174".
"6174 again!" She looked at Yu Chunsheng in surprise: "What's the principle behind this?"
“There’s no real principle behind it; that’s the charm of numbers,” Yu Chunsheng said with a smile. “The few digits he wrote down for you were actually arranged in ascending order, but you were so engrossed in the deeper thinking that you didn’t notice these superficial details.”
He then gestured on the draft paper to explain: "In fact, for any four-digit number, as long as they are not exactly the same, you can get the number 6174 in no more than seven calculations by following the approach I just described."
She never expected that this ordinary supply and marketing cooperative accountant would know so much. His already handsome face, coupled with his serious, focused, and meticulous demeanor, truly captivated Xu Tangmian.
She couldn't help but kiss Yu Chunsheng on the lips, abruptly interrupting what he was about to say.
Yu Chunsheng realized what was happening, stared intently at her smiling face, and couldn't help but smile along with her.
He suddenly realized something, quickly pushed Xu Tangmian away, stood up, wiped his nose, and said awkwardly, "You, you, try playing it yourself, it's quite interesting."
Xu Tangmian liked to flirt with him, and she would always run away after flirting. For her, watching Yu Chunsheng's helpless reaction was also a very interesting thing.
However, she didn't intend to make things difficult for him, so she let him off the hook and asked him without looking up, "What year were you born?"
Yu Chunsheng was in a state of confusion, and upon hearing this, he subconsciously answered her, "1946."
Xu Tangmian turned to look at him: "?"
"Uh... we're the same age as you, a few months older," he said, looking down again.
“Same year? That’s meaningless.” Xu Tangmian didn’t think much of it and wrote down “1999” on the paper again to calculate it.
A moment later, she exclaimed excitedly, "It really is! How did you know? Did you study it before?"
“We…taught a lot of things to our teachers when we were young.”
Xu Tangmian nodded without giving it much thought.
She wasn't from this era and didn't know about Yu Chunsheng's childhood curriculum, simply assuming that each place had its own customs. After all, Yu Chunsheng could speak Russian, couldn't she? Even in this new era, born in 1999, if she hadn't had opportunities to learn Russian in her daily life, she probably wouldn't have been able to speak a single word of Russian in her entire life.
"You're really amazing," she exclaimed sincerely, her gaze towards Yu Chunsheng becoming even more profound.
However, Yu Chunsheng was sitting on the bed with his head down, not having time to respond to her "heartfelt expression".
On the second day of math class, just five minutes into the lesson, Teacher Geng, Director Huang, Principal Ding, and Li Yutao, who had secretly switched classes with someone else, stood neatly in the back row of the classroom.
This class is crucial to her career. Xu Tangmian admits that she didn't like the job at first and her attitude was somewhat lax, but that doesn't mean she'll be kicked out of the school in disgrace.
She breathed a sigh of relief and calmly continued with her usual teaching approach, inviting several students to perform on stage in the form of a game.
When the students finally figured out the mysterious "6174" themselves, they were just as surprised as she had been the night before. They shouted excitedly, chattering like sparrows, eager to try it themselves.
Besides Teacher Geng, several teachers in the back row of the classroom exclaimed "Wow!" with genuine admiration. Even Principal Ding smiled and nodded in satisfaction.
Xu Tangmian finally felt relieved.
Seeing Teacher Geng's stern face, she remembered what Yu Chunsheng had told her: "Don't offend anyone, always leave room for maneuver." She clapped her hands twice and said loudly, "Class, please be quiet."
The students were completely impressed by her, and they all obediently sat down, waiting for her to continue.
Xu Tangmian waved her hand toward the back of the classroom and said with a smile, "Actually, this lesson was taught to me by Teacher Geng. Teacher Geng knows that I am not very good at math, so he gave me some special tutoring."
"I'll call this little game the '6174' black hole. Now we'll split into two teams, the two teams on the left led by me, and the two teams on the right guided by Teacher Geng. Let's see who can figure out '6174' first, okay?"
"Good!!" The students' shouts were like a torch, completely igniting the atmosphere in the classroom.
Under Principal Ding's teasing, Geng Hao scratched his head and took a step forward sheepishly.
Some of the bolder students from the two groups on the right got up from their seats and came over to pull him, complaining, "Teacher Geng, you're too slow! There's a little idiot in our group who can't do the math at all. If this keeps up, we're going to fall behind their team."
"Really?" Geng Hao followed him, bowing as he walked to the "little fool's" side, forcing a smile: "Is there something you can't calculate?"
...
After class ended and he returned to his office, Geng Hao said with a stiff neck, "You're not so bad, you even know this... But don't get cocky, I'll still be watching you. If I catch you not paying attention in class, I'll be the first to report you."
“I accept the challenge.” Xu Tangmian held her head high, her face beaming.
"Teacher Xu, come out for a moment." Principal Ding suddenly called out to her from outside.
"Your teaching style is quite different from other teachers, and I was a little skeptical at first. But looking at the students' grades and their feedback recently, you're doing a good job. Keep up the good work."
Xu Tangmian nodded modestly and replied, "I still have many shortcomings to learn. My students and I are both exploring and growing."
“It’s good that you’re maintaining this attitude.” Principal Ding looked at her approvingly, then changed the subject and said, “You’ve been asking me to approve your letter of introduction to go to Beiping, but the school is a bit overwhelmed lately. How about this, wait until closer to the New Year, when the students are on holiday, and I’ll approve it for you then. You can spend the New Year with your relatives in Beiping, and you can come back to school a few days later.”
"Thank you, Principal!" Xu Tangmian was so excited she almost jumped up. "Don't worry, I will definitely arrange my work properly!"
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