Find an Unknown Side Length Using Perimeter

Students will use a given perimeter and known side lengths to find a missing side length of a polygon. They will set up equations and use subtraction strategies to solve, working with rectangles and other polygons.

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Grade 3 · Perimeter of Polygons · question 1 of 3

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12 rungs. A star at the top.

P = 14 ft, ? = ___ ft

Your answer:

How it is taught

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Perimeter is the distance around a shape.

A square's four sides are equal.

Read it: Walk the four sides

You have a 3 by 5 frame of squares. Counting every edge one by one is slow. The frame shows 15 inside and 16 around.

Walk the four sides of the frame: 5, 5, 3, 3.
Add them: 5 + 5 + 3 + 3 = 16 around.
Keep that apart from the inside count, which is 15.

Counting every tiny edge unit works, but it is slow and you lose track at the corners.

The 15 inside is how much space; the 16 around is how long the fence is.

Opposite sides match: two 5s and two 3s, so you add each pair once.

Add the four sides to walk around; the squares inside answer a different question.

Saying the walk around is 15 because there are 15 squares inside.

The 15 is the inside. Walk the four sides: 5 + 5 + 3 + 3 = 16.

A rectangle has perimeter 24. Its long sides are 8. How long is each short side? — 4. 8 + 8 = 16. 24 − 16 = 8, and the 2 short sides share it: 4 each.

Quadrilateral, perimeter 24 m, sides 6, 5, 7, ?

Imagine a garden shaped like this. The whole way around — its perimeter — is twenty-four meters.

Three sides are labeled: six meters, five meters, and seven meters. But one side is missing!

Grade 3 · The mystery side of a garden

Remember: the perimeter is the SUM of every side length.

So six plus five plus seven, plus our mystery side, equals twenty-four.

Let's add the sides we know. Six plus five is eleven.

Eleven plus seven is eighteen.

Now subtract: twenty-four minus eighteen is six.

Six! That's our mystery side — six meters.

Check: six plus five plus seven plus six equals twenty-four. The missing side is six meters!

What you see

While they climb, your page writes one true line to ask about tonight, and a short note each Sunday.

MayaGrade 3 · Mathematics

Did you know?

A chessboard has 64 squares — share them out between the 8 rows and each row holds exactly 8.

Today’s goal: 1 star — one skill mastered, every rung of its ladder climbed
Climbing now: Divide within 100

An example student.

MayaGrade 3 · Mathematics

Maya earned 2 stars this week — Interpret multiplication as equal groups, which took 6 answers over 3 days before its star landed 3 days ago and Multiply by 6, 7, 8 and 9. Next up: Divide within 100.

An example week.

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