Let <em>V</em> be the volume of the tank. The inlet pipe fills the tank at a rate of
<em>V</em> / (5 hours) = 0.2<em>V</em> / hour
and the outlet pipe drains it at a rate of
<em>V</em> / (8 hours) = 0.125<em>V</em> / hour
With both valves open, the net rate of water entering the tank is
(0.2<em>V</em> - 0.125<em>V </em>) / hour = 0.075<em>V</em> / hour
If <em>t</em> is the time it takes for the tank to be full, then
(0.075<em>V</em> ) / hour • <em>t</em> = <em>V</em>
Solve for <em>t</em> :
<em>t</em> = <em>V</em> / ((0.075<em>V</em> ) / hour)
<em>t</em> = 1/0.075 hour
<em>t</em> ≈ 13.333 hours