Problem: light glows for every 15 seconds . how many max . times did it glow between 1 : 57 : 58 and 3 : 20 : 47 am .
Rationale: "time difference is 1 hr , 22 min , 49 sec = 4969 sec . so , light glows floor ( 4969 / 15 ) = 331 times . answer : e"
annotated_formula: divide(add(add(const_2, 47), multiply(add(20, add(const_2, const_60)), const_60)), 15)