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
A) 3 hours
B) 5 hours
C) 7 hours
D) 10 hours
E) 15 hours
Explanation:   (A+5 hours)'s 2 hour's work when opened =   1/6+1/4=5/12   (A+5 hours)′s 4 hour's work=5/12∗2   =5/6Remaining work = 1−5/6=1/6   Now, its A turn in 5 th hour 1/6 work will be done by A in 1 hourTotal time = 4+1= 
<gadget id="calculator">4+1</gadget>
<output>5</output>
5hours   Option 5 hours
<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 ?