6 minutes to fill a bathtub 10 minutes to drain how long?

Q: 6 minutes to fill a bathtub 10 minutes to drain how long?

Answer: 15 mintutes

The tap can fill 1/6 of a tub per minute. The drain can remove 1/10 of a tub’s water in one minute. So after t minutes, the amount of water (in fractions of a tub) in an initially empty tub is 
(1/6)t – (1/10)t 

Setting this equal to 1 (for 1 whole tub) gives 
(1/6)t – (1/10)t = 1 
30(1/6)t – 30(1/10)t = 30 
5t – 3t = 30 
2t = 30 
t = 15 minutes