Two pipes A and B can fill a tank in 6 hours and 4 hours respectively. If they are opened on alternate hours and if pipe A is opened first, in how many hours, the tank shall be full?  Choose the most appropriate option: A) 2hrs B) 4hrs C) 5hrs D) 6hrs E) 7hrs
A's work in 1 hour = 1/6   B's work in 1 hour = 1/4   A+B 2 hour's work when opened alternately = 1/6 + 1/4 = 5/12   A+B 4 hour's work when opened alternately = 10/12 = 5/6   Remaining part = 1-5/6 = 1/6   it is A's turn and 1/6 part is filled by A in 1 hour.   Total time taken to fill the tank = 4+1= 
<gadget id="calculator">4+1</gadget>
<output>5</output>
5hrs   Answer is 5hrs
<result>C</result>
Two pipes A and B can fill a tank in 6 hours and 4 hours respectively. If they are opened on alternate hours and if pipe A is opened first, in how many hours, the tank shall be full?