GMAT Quantitative Reasoning Practice Test 6

Let x equal the amount of time passed. Since the time remaining is $\frac{1}{4}$ of the time that has passed, this time can be represented as (1/4)x. Converting to decimal form may make this problem easier, so change $\left(\frac{1}{4}\right)\mathit{x}$ to $25\mathit{x}$ . Since 1x is the time passed and $.25\mathit{x}$ is the time remaining, then $1\mathit{x}+.25\mathit{x}$ is the total time. This is equal to $1.25\mathit{x}$ . To calculate the percent of the period that is over, use the proportion

Now set up a proportion using the time passed as the part and the total time for the class as the whole.

$\frac{1}{1.25}=\frac{\mathit{x}}{100}$
Cross-multiply to get $1.25\mathit{x}=100$.
Divide both sides by 1.25 .
$\frac{1.25\mathit{x}}{1.25}=\frac{100}{1.25}$
$\mathit{x}=80\%$
$80\%$ of the class period is over.
For this particular question, the number of minutes in the class period is not needed to solve the problem.