How to calculate the permutation and combination?

No, the molecule is the multiplication of two numbers decreasing from 5, which is 5 * 4; The denominator is two numbers increasing from 1, namely1* 2; So the result is 5 * 4 ÷ (1* 2) =10;

Similarly: C53 = 5 * 4 * 3 ÷ (1* 2 * 3) =10.

c54=5*4*3*2÷( 1*2*3*4)=5