<PRE><B>Question 321571</B>
if one outlet pipe can drain a tank in 24 hours while a second outlet pipe can drain the same tank in 36 hours, how long will it take to drain the tank if both pipes are open?
draining the tank is 1 job.
..
1 drain does the job in 24 hours
so it does 1/24 of the job in 1 hour.
..
second does the job in 36 hours
so it does 1/36 of the job in 1 hour
..
together they do 1/24+1/36 of the job in 1 hour
LCD = 72
= 3+2/72 
=5/72
5/72 0f the job they do together in 1 hour
1 job they do in 72/5 hours
=14.4 hours</PRE>