My Hogwarts has a serious problem.

Chapter 245, Section 242: What is a chicken? And what is a lotus root?

Following the haunting incident last semester, Ravenclaw College has recently encountered another major problem.

That morning, a fourth-year wizard hurriedly came out of the common room and, after having breakfast in the Great Hall, realized that he had forgotten to bring his book for the first Potions class.

Professor Snape was no kind professor. Forget about him forgiving you for forgetting your books. This young wizard had been at Hogwarts for four years, and Snape had never smiled at any of his students, only given them cold sneers and mocking remarks.

He could only rush back to the lounge, praying that the riddle on the door knocker hadn't been changed, or that someone with urgent business would give him an easier one.

But when he returned to the door of Ravenclaw's lounge, he was dumbfounded by the questions he heard today.

How many five-digit numbers can be formed using the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9, containing three odd digits and two even digits?

"Me."

The fourth-grade wizard stared stammeringly at the door knocker that seemed to be acting strangely, immediately wondering if he had misheard.

"Please repeat the question."

The doorknob eagle seemed quite impatient as it repeated the question, as if the young wizard had bothered it.

"What kind of question is this? A five-digit number? And what are odd and even numbers?" The little wizard was exasperated. "Please, Brother Eagle, I'm going to be late if I don't get my book out soon! Professor Snape will skin me alive and turn me into potion ingredients!"

However, the doorknob eagle completely ignored his request. There had been times in the past when young wizards encountered urgent matters but could not answer its questions, but the impartial doorknob eagle had never allowed them to pass without permission.

The little wizard had no choice but to grit his teeth, turn around, and run towards the classroom after checking the time.

He'd get scolded if he didn't bring his book, but if he was late and still hadn't brought it, he could hold a memorial banquet for his death anniversary next year.

After getting rid of the troublesome little wizard, the doorknob eagle was finally able to continue counting.

However, not long after, a new group of young wizards returned from the Great Hall.

They were third-year Ravenclaw students. Since they had no classes this morning, they could go straight back to their dormitories after having breakfast in the Great Hall and spend a leisurely half-day.

"Have you finished the homework Professor Lockhart assigned?" A third-grade girl asked the white-haired, expressionless boy next to her with a grin.

"It's done."

The little witch exclaimed in surprise, "So fast! Defense Against the Dark Arts class has a lot of homework. The professor says we have to fill at least fifteen inches of parchment. How much did you write?"

"18 inches."

The boy seemed rather indifferent, but the young wizards and witches around him were used to it; they had been classmates for three years and knew that Ian Campbell was like this to everyone.

Although Ian acted coldly, the little witch remained enthusiastic, chattering away.

"You're always the first to finish your homework. Who do you usually do it with? Monica and I were thinking of finding you to do our homework together yesterday, but we didn't see you in the break room."

Ian may be aloof, but he always answers questions.

"Lynn of Slytherin and Cedric of Hufflepuff."

"I heard you're friends with them, and that they're both top students in their respective colleges, with Lynn Bellock being number one in our class since freshman year."

When Ian heard what she said, he couldn't help but think of Lynn's recent performance, which was completely unworthy of him being Slytherin's best student and having been ranked first in his year for three consecutive years.

As they were chatting, the group arrived at the door of the Slytherin common room. The door knocker hawk, who was counting, had to stop when he saw them and impatiently asked Lynn's question again.

How many five-digit numbers can be formed using the digits 0, 1, 2, 3, 4, 5, 6, 7, 8, and 9, containing three odd digits and two even digits?

Ravenclaw's little wizard: ∑(￣□￣;)

"What kind of riddle is this?"

Why combine these numbers into a five-digit number?

What are odd numbers? And what are even numbers?

A group of young wizards suddenly became Muggles. They included pure-bloods, half-bloods, and those from Muggle families. The pure-blood and half-blood wizards had basically never attended Muggle primary schools, while those from Muggle families, even if they had attended primary school, were not yet at the level to learn this kind of knowledge.

Ian also heard the question. He came from a noble family and although he had never attended primary school, he had been taught at home by a private tutor since he was a child, so he at least knew what odd and even numbers were.

Looking at the bewildered little wizards around him, he could only explain to them.

Even numbers are numbers that are divisible by 2. There is a formula for odd numbers: any number that is equal to 2n+1 is odd.

These young wizards understood the most basic mathematical concepts; after all, they were Ravenclaw students, and they immediately grasped Ian's meaning.

"That is to say, numbers like 1, 3, 5, 7, and 9 are odd numbers, while numbers like 2, 4, 6, and 8 are even numbers."

"Numbers can be differentiated in this way?" The young wizard from a pure-blood family was quite surprised.

The Muggle-born wizard explained to him, "This is a classification and summary made by Muggle mathematicians. The doorknob question should also be a question about Muggle mathematics."

"Then what about 0? Is 0 an odd or even number?" Every young Ravenclaw wizard has a thirst for knowledge, and soon the young wizards were intrigued by mathematical concepts.

This didn't require Ian's explanation; a clever little wizard deduced it himself.

「0应该算是偶数，因为没有任何任何数可以满足2n+1=0，而0÷2=0是整除，所以0应该是偶数。」

What he said actually had a flaw: substituting negative one-half into the formula would give 0. However, Ian didn't remind them. For them, it was already quite an achievement to understand odd and even numbers in such a short time. They could leave negative numbers for later, lest they get confused.

"How many five-digit numbers contain three odd digits and two even digits? That's a huge range. If we were to count them one by one, we wouldn't finish even after three days."

"The door knocker won't present us with an unsolvable problem," one of the eagles calmly shared. "There must be an easy way to calculate this number quickly!"

They didn't have classes in the morning, so they weren't in a hurry. They gathered in front of the door and started thinking about what simple way to solve the problem.

(End of this chapter)