ACT Preparation / Unit 7 ACT 0627 worked example, 60 to 90 seconds
Finding the Whole From a Part
The idea
When the amount and the percent are known and the whole is not, work in three steps. First, identify the amount, the part given, and the percent it represents. Second, write the percent equation with the base unknown: amount = percent × base. Third, divide the amount by the decimal percent, then check by taking that percent of the answer.
In practice
Worked example: 42 students walked to school, 35% of the student body. How many students attend? Step two: 42 = 0.35 × base. Step three: base = 42 ÷ 0.35 = 120. Check: 35% of 120 is 42. Second: a sale price of $68 is 85% of the original price, so the original was 68 ÷ 0.85 = $80.
Where people slip
Multiplying instead of dividing is the error to check against: 35% of 42 is 14.7, smaller than the part, so it cannot be the whole.
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