Question 603733
Three water pipes are used to fill a swimming pool. The first pipe alone takes 4 hours to fill the pool, the second pipe alone takes 6 hours to fill the pool, while the third pipe alone takes 12 hours to fill the pool. If all three pipes are turned on at the same time, how long will it take to fill the pool?
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let x=hours it  takes to fill pool with all three pipes turned on
1/4 =work rate of first pipe
1/6 =work rate of first pipe
1/12 =work rate of first pipe
1/x =work rate of all three pipes turned on
..
sum of individual work rates=work rate working together.
1/4+1/6+1/12=1/x
3/12+2/12+1/12=1/x
6/12=1/x
x=2
hours it  takes to fill pool with all three pipes turned on=2