GRE Lifesaver: The Combined Work Formula

February 1, 2013
Boris Dvorkin

GRE BlogDo you hate questions like this?

BOB can eat a cheesecake in THREE minutes. His sister JENNY can eat a cheesecake in TEN minutes. How long will it take the two of them, eating TOGETHER, to eat three cheesecakes?

If you’re anything like I used to be, these problems drive you nuts. But I say “used to be” because after I started teaching for Kaplan, I learned that there is actually a dirt-simple plug-and-chug formula that’ll spit out the answer for you. It’s call the combined work formula, and it’s this:

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A student in my class last night asked, “Why weren’t we taught this in high school?” Why, indeed. When I learned this formula was real, I wanted to go find my high school math teachers and smack ‘em.

So Bob and Jenny’s cheesecake addiction problem isn’t a hair-puller after all. Just plug their speeds into the formula…

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…and remember that this is the GRE, so there’ll always be a trap. Here, remember that the question asks for the time to eat THREE cheesecakes, so multiply the above by 3:

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And there’s your answer. In case you missed it in high school, don’t miss it now: the combined work formula is a mighty addition to your arsenal on Test Day!

Boris Dvorkin Boris scored in the 99th percentile on the PSAT, was a National Merit finalist, and went on to earn two degrees from Case Western Reserve University. As a two-time Kaplan Teacher of the Year, Boris has helped many students achieve their goals and is known for his sense of humor in the classroom. When Boris isn’t helping students tackle tests, he loves playing strategy board games.

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