Lesson 4: Math is Explaining and Sharing5.MD.C.5

I can…

I can construct an argument using equations, drawings, or words, and use volume to defend a recommendation.

Key words

• argument — A claim backed by reasons that others can check.
• conjecture — A math idea you think is true but have not proved.
• counterexample — One example that shows it is not always true.
• volume — How much space is inside a solid shape.
• cubic unit — A cube with sides 1 unit long. It measures volume.

Steps

1. Suppose each large box is 8 inches long, 3 inches wide, and 10 inches tall. Its volume is 8 × 3 × 10 = 240 cubic inches, so three large boxes hold 240 × 3 = 720 cubic inches.
2. Suppose each jumbo box is 10 inches long, 4 inches wide, and 10 inches tall. Its volume is 10 × 4 × 10 = 400 cubic inches, so two jumbo boxes hold 400 × 2 = 800 cubic inches.
3. I recommend buying two jumbo boxes because 800 cubic inches is greater than 720 cubic inches — the total volume is greater, so there is more cereal for the same price.

Try it

Two brothers disagree about which of two phone plans is the better deal. Which way of settling it best fits the idea that math is explaining and sharing?

Check your answer

Each brother explains his reasoning, listens to the other, decides if the other's argument is convincing, and double-checks the math

Explaining and sharing means communicating your reasoning, listening to others' arguments, deciding if they are convincing, and checking that the calculations are accurate.

Sentence starter

I know ___ because ___.

Goal: I can explain my thinking with clear math vocabulary using the words argument, conjecture, and cubic units.