Pipe B that can fill a tank in an hour and pipe A that can fill the tank in half an hour are opened simultaneously when the tank is empty. Pipe A is shut 15 minutes before the tank overflows. When will the tank overflow? Pick one:  A) 32 mins  B) 35 mins  C) 40 mins  D) 30 mins  E) 36 mins
The last 15 minutes only pipe B 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 + 2= 
<gadget id="calculator"> 1 + 2</gadget>
<output>3</output>
3 tanks/hour, therefore to fill 3/4th of the tank they need (time)= (work)/(rate) = (3/4)/3 = 1/4 hours = 15 minutes.   Total time = 15 + 15= 
<gadget id="calculator">15 + 15</gadget>
<output>30</output>
30 minutes.   Answer: 30 mins
<result>D</result>
Pipe B that can fill a tank in an hour and pipe A that can fill the tank in half an hour are opened simultaneously when the tank is empty. Pipe A is shut 15 minutes before the tank overflows. When will the tank overflow?