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 $\frac{1}{4}x$ to $25x$ . Since 1x is the time passed and $.25x$ is the time remaining, then $1x + .25x$ is the total time. This is equal to $1.25x$ . To calculate the percent of the period that is over, use the proportion $\frac{\text{ part }}{\text{ whole }} = \% \frac{x}{100}$ Now set up a proportion using the time passed as the part and the total time for the class as the whole. $\frac{1}{1.25} = \frac{x}{100}$Cross-multiply to get $1.25 x = 100$.Divide both sides by 1.25 .$\frac{1.25 x}{1.25} = \frac{100}{1.25}$$x = 80 \%$$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.