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 boy walks at a constant speed V=3 m/s where V is his velocity 1 minutes has 60 seconds Thus we conclude during 1 minute he travels : 60 seconds × 3 meters/second = 180 meters
