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 correct choice from the following options
A) 4 hrs B) 5 hrs C) 7 hrs D) 9 hrs E) None
Solution   A's work in 1 hour	= 1/6.   B's work in 1 hour	= 1/4.   (A+B)'s 2 hour's work when opened alternately	= (1/6 + 1/4)   = 5/12.   (A+B)'s 4 hour's work when opened alternately	= 10/12   = 5/6.   Remaining part	= (1 - 5/6)   = 1/6.   Therefore, total tank to fill the tank	  (4+1)= 
<gadget id="calculator">  (4+1)</gadget>
<output>5</output>
5 hrs.   Answer 5 hrs
<result>B</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 ?