How to answer it
- Name the starting amount and the rate.
- Decide whether the rate adds or subtracts over time.
- Check the model with a simple value, like 0 or 1.
Worked example
An original question in the ACT format, already answered.
ACT MathWorked example
A pool holds 18,000 gallons and is being filled at 450 gallons per hour.
Which expression gives the number of gallons still needed to fill the pool after h hours?
- A18,000 − 450h
- B450h − 18,000
- C18,000h − 450
- D450(18,000 − h)
Answer: A. The amount still needed starts at 18,000 and drops by 450 each hour. At h = 0, A gives 18,000, which checks.
The most common trap
Writing the amount filled instead of the amount still needed. Reread what the expression should count.
Practice it
Hourly Prep’s daily sets mix modeling with the rest of ACT Math, and send more of it your way when it’s a weak spot. See how the ACT prep plan works, or browse the other ACT skill guides.