Question

How many 5-digit numbers can be formed (using 0-9) without repetition?

Answer 1

let's look at this with 4 numbers first if you can't repeat than you get
1234, 1243 1324 1342 1423 1432etc. basicly for each position you have 4 options on the first postion you can still pick each number so 4, on the second position you can only pick from 3 different numbers (since you can't repeat.) so eventually your amount of options is multiplied to:
4*3*2*1 = 24 options. (you can check this by writing it out.)

in the same way solve for 9numbers