A and B working saperately can do a piece of work in 12 and 18 hours respectively. If they work for 6 hour alternately with B beginning, in how many hours will the work be completed?  Options:
A) 9 hours B) 15 hours C) 13 ½ hours D) 12 ½ hours E) 16 hours
Explanation:   A and 15 hours 2 hours work = 1/12 + 1/18 = 5/36   work done by A and 15 hours during 6 pairs   i.e 12 hours = 6 * 5/36 = 5/6   Remaining work = 1 – 5/6 = 1/6   1/6 of work and this work done by 15 hours only = 18* 1/6= 
<gadget id="calculator">18* 1/6</gadget>
<output>3</output>
3 hours   Hence the total time take is 15 hours   Answer: Option 15 hours
<result>B</result>
A and B working saperately can do a piece of work in 12 and 18 hours respectively. If they work for 6 hour alternately with B beginning, in how many hours will the work be completed?