How long will it take for two pipes to fill a tank together when they can fill it alone in 14 hours and 21 hours respectively?

Question: How long will it take for two pipes to fill a tank together when they can fill it alone in 14 hours and 21 hours respectively?Solution:together in one hour they can fill = 1/14 + 1/21 = 5/42so, the total time to fill the tank is = 42/5 hours = 8.4 hours