Cogito
Grade 6 Math · Chapter 8 · Lesson 3
Finding a Missing Edge
Working backwards from the volume.
10 problems · about 17 minutes · 6.G.A.2
What this lesson teaches
The child finds an unknown edge length of a prism given its volume and two edges.
- Volume is the three edges multiplied together.
- Multiply the two known edges, then divide the volume by that.
- Check by multiplying all three back to the original volume.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Packing with Fraction Cubes: 32 half-unit cubes. Volume in cubic units?
Answer 4
Why 4 cubic units.
Review — Volume When the Edges Are Fractions: A box 8 by 3 by 1/2. Volume?
Answer 12
Why 12 cubic units.
Review — Breaking a Shape Apart: A 7 by 4 rectangle with a 3 by 2 piece removed. Area left?
Answer 22
Why 22 square units.
Warm Up
Straightforward practice. Get the method working first.
3 problemsVolume 72, length 6, width 3. Height?
Answer 4
Why 4.
Volume 24, base area 6. Missing edge?
Answer 4
Why 24 ÷ 6.
Volume 100, length 5, width 4. Height?
Answer 5
Why 100 ÷ 20.
Build It Up
The same ideas with more to keep track of.
2 problemsVolume 36, length 3, height 3. Width?
Answer 4
Why 36 ÷ 9.
Volume 2 by 3 by 7. What is it?
Answer 42
Why Multiply all three.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Tank: Volume 48, length 4, width 3. What is the height?
Answer 4
Why 4.
The Crate: Volume 90, length 5, height 3. What is the width?
Answer 6
Why 6.