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molly can solve this problem in minutes. paul needs 10 minutes to solve this problem. how long would it take them to solve this problem together?

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1 Answer

Assuming that you have the time that it takes Molly to complete the problem, the formula that you're going to use is:
 
(1/Molly's Time) + (1/Paul's Time) = (1/Time Working Together)
 
Think of each person's efforts as a proportion of the total job being represented by the two terms on the left side of the equation. The more time they take, the lower the proportion is of the total job that they do (the denominator gets larger as time increases). The two terms sum to give you the inverse of the total time that it takes for them to complete the job.
 
Hope this helps!