In 1941, a series of silver coins was introduced in denominations of 5, 10, and 20 satang, due to a shortage of nickel caused by World War II. The next year, tin coins were introduced for 1, 5, and 10 satang, followed by 20 satang in 1945 and 25 and 50 satang in 1946. In 1950, aluminium-bronze 5, 10, 25, and 50 satang were introduced whilst, in 1957, bronze 5 and 10 satang were issued, along with 1 baht coins struck in an unusual alloy of copper, nickel, silver, and zinc. Several Thai coins were issued for many years without changing the date. These include the tin 1942 1 satang and the 1950 5 and 10 satang, struck until 1973, the tin 1946 25 satang struck until 1964, the tin 50 satang struck until 1957, and the aluminium bronze 1957 5, 10, 25, and 50 satang struck until the 1970s. Cupronickel 1 baht coins were introduced in 1962 and struck without date change until 1982.
Question: For how many years did the tin 50 satang coin remain without a date change?
Answer: 15