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?  Pick one
A) 20 1/2 hours
B) 20 1/7 hours
C) 20 1/3 hours
D) 20 3/2 hours
E) 20 1/4 hours
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   20 1/2 hoursnswer: 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?