permutations help!

Question

1.how many 4digit numbers can be formed with

a) repetition and b) without it

Answer 1

b) without repetition: 10!/(10-4)! =10!/6! = 10x9x8x7x6!/6! = 10x9x8x7 =90x56 = 5,004

a) with repetition: 10^4 =10x10x10x10 = 10,000