The us fifty cent piece decreased the silver composition in 1965. The center of the coin is a copper alloy. The coin keeps the silvery appearance due to the coating of 90.0% silver on the outside of the coin. Due to decrease of popularity of the use of the coin the last major minting occurred in 1970. The average percent of silver in a half dollar coin is 36.80%. The average mass of a 1970 half dollar is 11.50 grams. If you need 21.6 grams of silver for an experiment and the only source of silver available are 1970 half dollars, how many coins do would you need?