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 options:
A) 20 1/2 hours
B) 20 1/7 hours
C) 22 1/2 hours
D) 20 1/3 hours
E) 20 3/2 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?