Two pipes A and B can fill a tank in 36 min. and 45 min. respectively. A water pipe C can empty the tank in 30 min. First A and B are opened. After 7 minutes, C is also opened. In how much time, the tank is full?
Choose the correct answer
A) 43 min  B) 44 min  C) 45 min  D) 46 min  E) 47 min
part filled in 7 min= 7*(1/36+1/45) =7/20   remaining part= 1-7/20 =13/20   net part filled by pipes A,B,C=(1/36+1/45-1/30) =1/60 (in 1 min.)   13/20 part is filled in = 60*13/20= 
<gadget id="calculator">60*13/20</gadget>
<output>39</output>
39 min   total time taken to fill the tank= 39+7= 
<gadget id="calculator">39+7</gadget>
<output>46</output>
46 min   ans= 46 min.   ANSWER:46 min
<result>D</result>
Two pipes A and B can fill a tank in 36 min. and 45 min. respectively. A water pipe C can empty the tank in 30 min. First A and B are opened. After 7 minutes, C is also opened. In how much time, the tank is full?