Question

at 10:00 AM pipe A began to fill an empty storage tank. At noon, pipe A failed and was closed. Pipe B was used to finish filling the tank. If pipe A needs 6 hours to fill the tank alone and pipe B needs 8 hours, at what time was the tank full?

Answer 1

let 1 represent the task of completely filling the tank
rate of pipe A = 1/6
rate of pipe B = 1/8
combined rate = 1/6 + 1/8 = 7/24
After 2 hours tank was filled 2(7/24) or 7/12
which remains 5/12 is yet to be filled
t(1/8) = 5/12
t = 8(5/12) = 10/3 hours or 3 hours and 20 minutes needed after noon

so at 3:20 the tank is full