What is 15' in maths?

Answer 1

15' can mean a length of 15 feet;

15' can also mean a length of time of 15 minutes;

15' can also mean an angle of 15 minutes (1 minute is 1/60 degree → 15' = 15/60 = 1/4 = 0.25°).

It all depends upon the context to give the probable unit for the ' mark.