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Interesting mathematics: pigeon hole principle ~ how to ensure that the difference between at least two cards is equal to 4?
question

There are 24 cards in the drawer, and the numbers on the cards range from 1 to 24. How many cards do you need to take out to ensure that the difference between at least two cards is equal to 4?

analyse

This question can be reversed: if the difference in the number of cards is not equal to 4, how many cards can you take out at most?

So we can divide 1 to 24 into four groups by dividing the remainder by 4:.

The numbers of different groups are definitely not equal to 4.

In each group can be divided into two groups, so * * * has eight groups:

In order to find the number of business trips not equal to 4, you can choose the following four groups:; * * * Yes 12 (brand).

If you take another card, it must belong to one of the following four groups: B0, B 1, B2, B3.

In any case, there will be a card with a difference of 4.

The conclusion is that at least 13 cards can be taken out to ensure that there will be cards with a difference of 4.