Calculation method:

Calculate the product of the first two numbers from 10: 10*9=90( 10 number).

Calculate the product of two numbers from 1: 2* 1=2 (two out of two).

The quotient of two numbers is the number of groups: 90/2=45 groups.

The calculation formula of two groups of students in two middle schools: x=? n(n- 1)/2

Double entry is a common method in lottery betting, which has a fixed calculation formula.

N(n- 1)/2 = the number of two groups.

For example, the number 10 is 10× 9 ÷ 2 = 45 groups.

The number of groups of n (n-1) (n-2)/3 * 2 *1= two out of three (or three out of three)?

For example, the number 10 is the group 10 * 9 * 8 ÷ 6 = 120.

N (n-1) (n-2) (n-3)/4 * 3 * 2 *1= four out of four (or two out of four) groups.

For example, the number 10 is 10 * 9 * 8 * 7 ÷ 24 = 2 10 group! ?

n(n- 1)(n-2)(n-3)(n-4)/5*？ 4*? 3*2? * 1= number of groups of five (or two in a group of five)

For example, the number 10 is10 * 9 * 8 * 7 * 6 ÷120 = 252 groups.

N (n-1) (n-2) (n-3) (n-4) (n-5)/6 * 5 * 4 * 3 * 2 *1= six out of six (or six out of two) groups.

For example, the number 10 is10 * 9 * 8 * 7 * 6 * 5 ÷ 720 = 210 group.