Worked Example 1
Missing Side From Area
A rectangle has an area of 72 square feet and a width of 6 feet. Find the length, l.
- 1Area = length × width: 6 × l = 72
- 272 ÷ 6 = 12
Answer: l = 12 feet
Florida Grade 4 Math · Geometric Reasoning
MA.4.GR.2.1: Perimeter and Area With Unknown Sides
What this Grade 4 benchmark means, worked examples, common mistakes, and a free printable worksheet.
Created by a Florida mom · Published by Tropical Learning LLC
What Is Benchmark MA.4.GR.2.1?
Students solve perimeter and area problems for rectangles with whole-number side lengths, including problems where a side length is unknown. The unknown is shown with a letter.
Area = length × width. Perimeter is the sum of all four sides. If the area and one side are known, divide to find the other side. Answers use units in word form (feet, square feet).
Official benchmarkSolve perimeter and area mathematical and real-world problems, including problems with unknown sides, for rectangles with whole-number side lengths.
On the FAST test: part of the Grade 4 reporting category Geometric Reasoning, Measurement, and Data Analysis and Probability, which is about 31–37% of the test.
Worked Examples
Worked Example 2
Missing Side From Perimeter
A rectangle has a perimeter of 50 meters and a length of 15 meters. Find the width.
- 1Two lengths: 15 + 15 = 30
- 2The two widths make the rest: 50 − 30 = 20
- 3One width: 20 ÷ 2 = 10
Answer: 10 meters
Common Mistakes to Watch For
Mixing up area and perimeter
Wrong:A 6 ft by 12 ft rectangle has a perimeter of 72 feet.
72 square feet is the area. The perimeter is 6 + 12 + 6 + 12 = 36 feet.
Subtracting only one length
Wrong:Perimeter 50 m, length 15 m: width = 50 − 15 = 35 m
There are two lengths and two widths. 50 − 15 − 15 = 20, and 20 ÷ 2 = 10 meters.
Helpful Math Tip
Sketch the rectangle and label every side you know. Seeing all four sides prevents most perimeter mistakes.
Related Grade 4 Skills
Make Florida Math Practice More Fun
Help your child build math confidence with original interactive questions aligned with Florida's B.E.S.T. Mathematics Standards.
Start Practicing FreeIndependent Educational Platform. FastChomp is published by Tropical Learning LLC and is not affiliated with, sponsored by, or endorsed by the Florida Department of Education. FAST and Star assessment names are referenced for informational purposes only. FastChomp practice questions are original educational materials, not official Florida FAST assessment questions.