Task: Question Answering
Input (English): Problem: a certain airline ' s fleet consisted of 90 type a planes at the beginning of 1980. at the end of each year, starting with 1980, the airline retired 3 of the type a planes and acquired 4 new type b plans. how many years did it take before the number of type a planes left in the airline ' s fleet was less than 50 percent of the fleet ?
Options: a. 11, b. 12, c. 13, d. 14, e. 15
Output (English): c