GRE Quant online: answer the requested quantity in a rates problem
A correct calculation can still answer the wrong question. In rate problems, write down the starting amount, target amount and net rate before dividing. That makes the requested quantity visible.
Original example with complete conditions
An initially empty tank has a capacity of 100 gallons. Water enters at a constant 8 gallons per minute while a drain removes water at a constant 3 gallons per minute. Both operate throughout the filling process. How many minutes does it take to reach 80% of capacity?
The net filling rate is 8 − 3 = 5 gallons per minute. The target volume is 0.80 × 100 = 80 gallons. Starting from empty, the time is 80 ÷ 5 = 16 minutes.
The simultaneous inflow and outflow are an idealised mathematical model specified by the problem. Do not add a delay or a changing rate that the conditions do not give.
Compare the plausible wrong quantities
- 5 is the net rate, not a time.
- 10 minutes would fill 80 gallons at the inflow rate alone, ignoring the drain.
- 20 minutes is the time to reach the full 100 gallons, not 80% capacity.
Check the correct answer by multiplication: 16 minutes × 5 gallons per minute = 80 gallons. Units help distinguish a rate, a volume and a time.
Apply the same check elsewhere
In a percent problem, label the reference amount. In a probability question, distinguish “exactly” from “at least”. In a comparison, identify every allowed value. These checks should respond to an actual error; they are not a ritual that every question must take the same time to complete.
Use ETS’s Quant overview for the real question formats and official practice for broader preparation. A short worked example does not establish a predicted GRE score.
Bring your first setup to an online GRE tutoring lesson. The useful question is which condition your model missed and how a fresh exercise will test the correction.