You've got 20 couples and you need to pick people so no two can be from a couple.

Easy, you have 40 choices for the first committee member... you can pick anyone you like.

Then, out of the reamaining 39 people, you cannot pick the other half of the couple which you just picked. So you have 38 choices.

Once you pick out 2 people that aren't from the same couple, you've immediately ruled out their other half as well, so you can only choose from the remaining 18 couples, therefore 36 choices.