Combined Rate of Two Machines

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It would take one machine 7 hours to complete a large production order and another machine 5 hours to complete the same order. If they can work on the order simultaneously without interfering with each other, how many hours would it take both machines, working simultaneously at their respective constant rates, to complete the order?

Review: Combined Rate of Two Machines


Explanation

The rates of these two machines, as measured in units of orders per hour, add to give the combined rate of their working together.









This value doesn't pop out in the answer choices, but



The correct answer is (C).

Many people prefer to memorize the formula for what we have just done as



This formula is swift to use, but I recommend working with rates directly to be able to handle more difficult questions, and also to let units guide you and prevent accidentally flipping a fraction.

Again, the correct answer is (C).


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