Answer:
15/2 or 7.5 minutes
Explanation:
Tap 1 fill a part of tank in 1 minute = 1/20
Tap 2 fill a part of tank in 1 minute = 1/12
2 tap of tank fill in 1 minute = 1/12 + 1/20
= 2/ 15
So if both tap open, the tank will be filled in 1/(2/15) = 15/2 or 7.5 mins
Hope this help :3