SME
Grade 3 Math · Problem type
Perimeter and unknown side lengths
Use this for “Three sides are 9 cm, 6 cm and 7 cm. The perimeter is 30 cm. How long is the fourth side?”
Diagram
Perimeter: 26 cm all the way around
This is an example. Type your own values to start a new problem.
Side 1 (cm)
+
Side 2 (cm)
+
Side 3 (cm)
+
Side 4 (cm)
=
Perimeter (cm)
Formulas
8 + 5 + 6 + 7 = 26
Side 1 + side 2 + side 3 + side 4 = perimeter
Assumptions
•
Add all the sides to get the perimeter.
•
To find one side, take the other sides away from the perimeter.
Step-by-step
WE KNOW
Perimeter: 26 cm
Side 1: 8 cm
Side 2: 5 cm
Side 3: 6 cm
FIND
side 4
Step 1 · Find side 4
8 + 5 + 6 + ? = 26
Side 1 + side 2 + side 3 + side 4 = perimeter
Add the sides you know. Take them away from the perimeter.
Side 4 = 26 − 8 − 5 − 6
8 + 5 = 13
13 + 6 = 19
26 − 19 = 7
Side 4 = 7 cm
Check
Put the numbers back in. Both sides should match.
8 + 5 + 6 + 7 = 26 ✓
Topic check
See what you know: 5 questions on this topic
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