40% + 35% = 75% of the coins were either made before 1940, or made from 1940 to 1980. In other words, 75% of the coins were minted before 1980. This leaves 100% - 75% = 25% of the coins to being minted after 1980.
25% of 1600 = 0.25*1600 = 400 coins were made after 1980
