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.