It all comes down to F / Everything becomes F-the Perfect insider (Subete ga F ni Naru: The Perfect Insider) (№ 238252)

Seven is a special number. It is the only lonely number.
- What do you mean?
- Try to divide the numbers from one to ten into two groups. And then multiply all the numbers inside the groups. Is it possible to make it so that these two works are equal?
- It is impossible. One of the groups will contain seven, which means that the product will be a multiple of seven. However, in another group of seven, and the works may not be equal.
- See? This is why seven is a lonely number. And B with D.