GMAT Practice Question Source:
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GMAT Timing Tips:
Rate problems: Use D = R x T and W = R x T
In this question, it’s probably most useful to use the variations R = W/T and T = W/R, because you are given 2 rates (A + B = 800/x and A = 800/y), and you really need to solve for B, because the question asks you for the time taken by Machine B, which is 800/B. You can subtract the second rate equation from the first rate equation to solve for B, then simplify this and plug it into 800/B to get the answer.
Set the amount of work equal to 1 for a single job
Because the amount of work done (800 nails) is always the same, we can simplify our calculations even more by just calling this a single job and setting the amount of work equal to 1, instead of 800.
Add rates when they are simultaneous
When we are talking about two simultaneous rates, such as Machines A and B working together, it’s important to realize that we can add the rates to get their combined rate. This is what allows us to say that the combined rate is A+B, if the rates for each machine are A and B.
Pick smart numbers to plug into variables in answer choices
When you see that the answer choices contain variables, an alternative approach is to choose smart numbers to plug in for each variable. You also need to know what value you need to get after you plug in the numbers, so that you can eliminate any answer choices that don’t give you that value. One really easy scenario to use is one in which each machine produces 400 nails per hour (this is what Bunuel used in this post on GMAT Club), in which case x = 1 and y = 2, and the final value needs to be 2 hours. When you plug x = 1 and y = 2 into the answer choices, E is the only one that given an value of 2 hours, so that is the correct answer. Note that, if more than one answer choice gave you the correct value, you would need to choose another set of numbers and plug them into the remaining answer choices.
Have a question that you want Jeff to answer about this GMAT practice question?
Ask your question in any of the following ways:
- GMAT Club: See the full question and ask your question in this thread: Working simultaneously at their respective constant rates, Machines A and B…