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All courses Math Foundations · MF.PR.8 22 of 60
Use proportional reasoning to solve percent problems without relying on memorized templates alone.

Find a part from a percent and whole

Problem
Use a percent proportion to find \(35\%\) of \(80\).
Your answer
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Hint

Let x be the part you want, then write the proportion x/80 = 35/100.

Percent means "out of 100," so use part/whole = percent/100.

Solution walkthrough

01

Write the percent proportion

\[\text{part}/\text{whole}~=~\text{percent}/100~⇒~x/80~=~35/100\]

The unknown x is the part, 80 is the whole, and 35% means 35 out of 100.

02

Solve for the part

\[100x~=~80(35)~=~2800;~x~=~28\]

Cross multiplication followed by division by 100 finds the requested part.

03

Check and answer

\[28\]

The decimal check 0.35 × 80 = 28 confirms the result.

+

Another way

  1. Convert 35% to 0.35 and multiply 0.35 by 80.

!

Common mistake

Use 35/100, not 35 as a multiplier. Percent means out of one hundred.

Solution walkthrough video