When you go to select the 2 pair to take ...
-- The first one can be any one of the 7 you own. For each if these . . .
-- The second one can be any one of the remaining 6 you own.
-- So there are (7 x 6) = 42 ways to choose 2 pairs of jeans.
But . . . . .
When you finish packing the 2 pairs of jeans you chose, it doesn't
matter which one was chosen first and which one was chosen last.
The same jeans could have been chosen for the trip in two
different ways.
Although there are 42 ways to choose two pair, the same two pair
can be chosen in 2 different ways. So there are only 42/2 = 21
different sets of jeans that can end up going on vacation.
