one pipe can fill a tank in 15 hour.but because of hole in a tank,this tank fill in 20 hour.so in how much time this hole will empty the full tank?  Pick one: A) 30 B) 40 C) 50 D) 60 E) 70

Rationale: in 1 hour hole empty = [ 1 / 15 - 1 / 20 ] = 1 / 60 so total time taken to empty the full tank through hole = 60 hour . answer d
Answer: D