Part of tank filled by first tap in one hour = 1/10
Part of tank filled by second tap in one hour = 1/15
Part of tank filled by first and second tap in one hour = 1/10 + 1/15 = 3/30 + 2/30 = 5/30 = 1/6.
So the tank will be filled in 6 hours when both taps are turned on simultaneously.