The question is

How many integers are between 0 and 40? my answer 41

How many of these integers are divisible by 2? my anser 20

How many unordered pairs of these integers are whose difference is 5?

Here I do not know where to start? An unordered pair is {#1, #2}, so I think I have two sets of numbers

set one are all numbers possible for #1 and

set two are all numbers possible for #2

First I am not sure how to treat the sets, because the pairs are unordered. So, What I understand is that in the first place could be a 30 and in the second a 5 or the other way around.

Then, I do not know how to express the "difference is 5"

Thanks for any help.