Two pipes X and Y can separately fill a cistern in 18 and 24 hours respectively. If they are turned on alternately for one hour each, how long will it take to fill the cistern?  Choose the correct choice from the following answers:
A) 20 hours 30 min B) 25 hours 30 min C) 30 hours 30 min D) 40 hours 30 min E) 50 hours 30 min
20 hours 30 min   20 hours 30 min   1/18 + 1/24 = 7/72   72/7 = 10 2/7   7/72 * 10 = 35/36 ----- 20 hours   WR = 1 – 35/36 = 1/36   1 h ----- 1/18   ? ----- 1/36 = 1/2 hours   =&gt; 20 1/2 hours
<result>A</result>
Two pipes X and Y can separately fill a cistern in 18 and 24 hours respectively. If they are turned on alternately for one hour each, how long will it take to fill the cistern?