Pipe A that can fill a tank in an hour and pipe B that can fill the tank in 15 are opened simultaneously when the tank is empty. Pipe B is shut 15 minutes before the tank overflows. When will the tank overflow?
Select the correct option:  A) 40 mins  B) 35 mins  C) 30 mins  D) 32 mins  E) 36 mins
The last 15 minutes only pipe A was open. Since it needs 1 hour to fill the tank, then in 15 minutes it fills 1/4th of the tank, thus 3/4 of the tank is filled with both pipes open.   The combined rate of two pipes is  1 + 4= 
<gadget id="calculator"> 1 + 4</gadget>
<output>5</output>
5 tanks/hour, therefore to fill 3/4th of the tank they need (time)= (work)/(rate) = (3/4)/5 = 3/20 hours = 15 minutes.   Total time = 15 + 15= 
<gadget id="calculator">15 + 15</gadget>
<output>30</output>
30 minutes.   Answer: 30 mins
<result>C</result>
Pipe A that can fill a tank in an hour and pipe B that can fill the tank in 15 are opened simultaneously when the tank is empty. Pipe B is shut 15 minutes before the tank overflows. When will the tank overflow?