Pipes A and B can fill a tank in 10 hours and 15 hours respectively. Both together can fill it in?

Part filled by A in 1 hour = 1/10
Part filled by B in 1 hours = 1/15
Part filled by (A + B) in 1 hour = (1/10 + 1/15) = 5/30 = 1/6
∴ Both pipes together can fill the tank in 6 hours.