I was shown how to do this question but I am not sure why it was done like this.

Four numbers are drawn at random from a box of ten numbers 0,1,...,9.

Find the probability that the largest number drawn is a six with replacement.

S={0,1,2,3,4,5,6,7,8,9} #S=10

A:Set of ordered 4-tuples out of S_1={0,1,2,3,4,5,6} #S_1=7

B:Set of ordered 4 tuples out of S_2={0,1,2,3,4,5} #S_2=6

Then,

P(largest of four numbers drawn with replacement is 6)=

[#(A)-#(B)]/10^4, =[(7^4)-(6^4)]/10^4

What I don't understand is why is it [#(A)-#(B)].