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.
In a regular polygon, each exterior angle equals each central angle. central angle = 360 / #sides #sides = 360 / central angle #sides = 360 / 60 #sides = 6
