Two pipes A and B can fill a tank in 36 hours and 45 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?
Select
A) 20 hrs
B) 22 hrs
C) 23 hrs
D) 24 hrs
E) 21 hrs
1/( 1/36 +1/45) = 180/9= 
<gadget id="calculator">180/9</gadget>
<output>20</output>
20 hrs   20 hrsNSWER:20 hrs
<result>A</result>
Two pipes A and B can fill a tank in 36 hours and 45 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?