<PRE><B>Question 253212</B>
Tank is full = 1
Tank is empty = 0
Rate of filling is t/30
Rate of emptying is t/45
The condition is therefore 1-(t/30) + (t/45) = 0
LCM is 90 so 90/90 -(3t - 2t)/90 = 0
or 90 -(3t - 2t)=0
90 -t = 0
t = 90 (minutes)
The problem gets more difficult if the rate of emptying depends on the level of water left in the tank, but this is not the case here.</PRE>