Three taps A, B ,& C can fill a tank in 12, 15, & 20 hours respectively. If A is open all the tank and B & C are open for one hour each alternatively,the tank will be full in:
A -- 1 tank/ 12 h = .08333 tank/hr
B -- 1 tank/ 15 h = .06666 tank/hr
C -- 1 tank/ 20 h = .05000 tank/hr
time t hours
.08333 t + .06666(t/2) +.05000(t/2) = 1 tank
.14167 t = 1
t = 7.06 hours