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20 letters are divided into two containers. How many combinations are there?
I'll tell you how to calculate 4, and 20 can be analogized.

First of all, you don't distinguish between two boxes, at least put 1 letters.

There may be 1 box A and three boxes B.

Or two boxes of a and two boxes of B.

In fact, it is 4 ÷ 2 = 2.

If it is 5, 5 ÷ 2 = 2.5, round it to 2.

Namely A 1 B4 and A2 B3 (A and B are indistinguishable).

Then do the calculation.

In the case of four letters, there are 1 in box A and three in box B, that is, "C4 1" (I don't know if you have learned A and C, C4 1 refers to randomly taking 1 from four elements, and the calculation formula is cab = (a× (a-6544)

Then there are two boxes A and two boxes B, which means C42 = 6. Because AB is indistinguishable, dividing by 2 is 3. Similarly, when the number of letters in the AB box is the same, this calculation is repeated, so it is divided by 2.

4+3=7

So four letters is seven.

There are 20 letters, that is C201+C20 2+...+(C2010 ÷ 2) = 20+190+... I won't count the rest. .