Cogito
Grade 3 Math · Chapter 4 · Lesson 3
Finding a Missing Side
Working backwards from the area.
10 problems · about 12 minutes · 3.MD.C.7
What this lesson teaches
The child finds an unknown side length of a rectangle given its area and one side.
- Area is one side times the other.
- A missing side is a missing factor.
- Divide the area by the side you know.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Area by Multiplying: A rectangle 8 by 7. What is its area?
Answer 56
Why The area is 56 square units.
Review — What Area Is: A rectangle 7 across and 3 down. What is its area?
Answer 21
Why The area is 21 square units.
Review — Two-Step Problems With a Letter: 8 × 5 − 14 = ?
Answer 26
Why 8 × 5 = 40, and 40 − 14 = 26.
Warm Up
Straightforward practice. Get the method working first.
3 problemsArea 42, one side 6. What is the other side?
Answer 7
Why The other side is 7.
What kind of question is finding a missing side?
Answer A missing factor question.
Why Area is a product, so a missing side is a missing factor.
Area 24, one side 6. What is the other side?
Answer 4
Why 24 ÷ 6.
Build It Up
The same ideas with more to keep track of.
1 problemArea 45, one side 9. What is the other side?
Answer 5
Why 45 ÷ 9.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsArea 56, one side 7. What is the other side?
Answer 8
Why 56 ÷ 7.
The Garden Bed: A bed has an area of 32 square metres and is 4 metres wide. How long is it?
Answer 8 metres
Why 32 ÷ 4 = 8 metres.
The Sticker: A sticker has an area of 18 square cm and is 3 cm tall. How wide is it?
Answer 6 cm
Why 18 ÷ 3 = 6 cm.