Question 1135838
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Here is an alternative method for working these "working together" problems that you might like to try.  Many students like this method because it avoids using fractions.<br>
Consider the two pumps and several pools of the given size, and look at the least common multiple of the times it takes each of the two pumps.<br>
The LCM of 4 and 6 is 12.  In 12 hours, the new pump could fill 12/4 = 3 pools; in 12 hours, the old pump could fill 12/6 = 2 pools.<br>
So in 12 hours the two pumps together could fill 3+2 = 5 pools; therefore the time required by the two together to fill one pool is one-fifth of 12 hours, or 12/5 hours.