Four groups of numbers from 0 to 9, each group randomly selects a math to form a group of four numbers, so how many such groups are there? - Training Enrollment Network

Four groups of numbers from 0 to 9, each group randomly selects a math to form a group of four numbers, so how many such groups are there?

Solution, four different numbers are, n1= c (10,4 4) = 2 10/0 (species)

One is similar, two are different, N2 =10c (9,2) = 360 (species).

The same is true for n3 = c (10/0,2) = 45 (species).

There are three identical and one different, n4 = a (10/0,2) = 90 (species).

4 same, n5= 10 (species)

Then n = 210+360+45+90+10 = 715 (species).

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