Watch with Theo · Calculator decisions
ACT Math Calculator Strategy: Set Up the Problem First
See why calculating the discount is different from calculating the sale price.
Take this into your next practice session
Write the setup, calculate, and check whether the result makes sense in context.
Choose an answer, then check your reasoning. This public practice is not saved.
Read all answer explanations
$18: Find 15% of $120 and subtract that discount, or multiply $120 by the 85% you still pay.
$105: Find 15% of $120 and subtract that discount, or multiply $120 by the 85% you still pay.
$138: Find 15% of $120 and subtract that discount, or multiply $120 by the 85% you still pay.
$102 — correct answer: Correct. The discount is $18, so the sale price is $120 − $18 = $102. Equivalently, 120 × 0.85 = 102.
Full transcript and worked example
Give your calculator a plan.
Hi, I'm Theo. A calculator can do arithmetic, but you still have to decide what to calculate. A jacket costs eighty dollars and is twenty five percent off. What do you pay before tax?
$80 jacket · 25% discount
Name what you are finding.
Eighty times point two five equals twenty. That is the amount saved. If you select twenty as the sale price, you stop one step too soon.
80 × 0.25 = 20
Calculate the amount left.
Subtract the twenty dollar discount from eighty. You pay sixty dollars. Another method is to multiply eighty by point seven five, because you are paying seventy five percent of the original price.
80 − 20 = 60
Estimate before you trust it.
A sale price should be less than the original price. That quick check can catch a mistyped calculation. Write your setup first, then use a permitted calculator you have practiced with.
$60 is less than $80
Your turn.
An item costs one hundred twenty dollars and is fifteen percent off. What is its sale price before tax? Decide whether your calculation gives the discount or the amount you actually pay.
$120 item · 15% discount
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