Well 42 divided by 2 is 21. than to double check you can do 21 plus 21 equals 42.
Number of ways of picking a smaller number out of a bigger number is called combination.
Number of way of picking 3 stamps out of 4 is 4C3 (4 combination 3) = 4! / (3! x (4 - 3)!) = 4! / (3! x 1!) = (4 x 3 x 2 x 1) / (3 x 2 x 1 x 1) = 24 / 6 = 4 ways.
The answer is 1/4 because she used 3/4 of the paint on 3 out of 4 walls.
