Question 1180836
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There are 20 even numbers (0,2,4,...,38) and 20 odd numbers (1,3,...,39)

There are two cases: the numbers are all odd or they are all even.  You will need to add them together:

If the numbers are odd, you get 20*19*18
If the numbers are even, you get 20*19*18

So the total number of combinations is  2(20*19*18) = 13680