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Source: Official Guide for the GMAT 13th Ed. Problem Solving; #58 Official Guide for the GMAT 2015 14th Ed. Problem Solving; #58


A glass was filled with 10 ounces

A glass was filled with 10 ounces of water, and 0.01 ounce of the water evaporated each day during a 20-day period. What percent of the original amount of water evaporated during this period?

2 Explanations


Imtisal A Siddiqui

By multiplying 0.1*20 we get 0.20 & then for finding out the %, we can multiply 0.20*10 to get 2 which means 2% of total amount!

Jan 21, 2018 • Comment

David Recine

While that solve does work in this case, I recommend against approaching the problem this way. It's not clear why you'd change 0.01 to 0.1. Moreover, the only reason multiplying 0.20 by the number of ounces works here is that we have 10 ounces. If we had-- for example-- 12 ounces instead, 0.2/12 would get us 1.7% (the correct answer), while 0.2*12 would get us 2.4-- not the correct percent!

It's better to instead multiply the original 0.01 by 20, to get 0.2, and then divide 0.2 by 10 to find the percentage. This approach can be seen in Mike's explanation video.

Jan 22, 2018 • Reply


Gravatar Mike McGarry, Magoosh Tutor

Dec 27, 2013 • Comment

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