Isha2701
Bunuel - what does without replacement mean here? I understand that we choose 10 and 20 def, meaning 1/20 x 1/19 x 18/18 x 17/17 x 16/16 (then for rest three numbers bcause we have 18 left now and any 18 can be chosen, similarly for 17 and 16 left.) And they all can be arranged in 5! ways..so where am I wrong?
“Without replacement” means that once a number is chosen, it is removed from the pool, so it cannot be chosen again and the next denominator drops from 20 to 19, then 18, and so on. Your mistake is using 5!: the three “any” positions are not distinct, so swapping those among themselves does not create new cases, which is why you count only 5!/3! = 20 arrangements, not 120.
For more please review the rest of the discussion.