Week 04 · Missing numbers

Thursday

The great acorn count

// Unknowns, turn-arounds and break-aparts

⏱ about 15 min

Thursday: The great acorn count

Chris the Coyote leans back against a pine tree.

"One fall, Samantha planted acorns in 8 rows of 7," he says.

"But she did not know 8 times 7 yet."

"Bennie showed her a trick. He laid a stick across the rows."

"Now there were two small arrays instead of one big one."

"Samantha knew both small facts, and she found the answer!"

Samantha's garden has 8 rows of 7. A stick splits each row into 5 and 2.

$A rectangle of unit squares, eight rows of seven, split into 5 and 2 columns$

Break 7 into 5 and 2.

8 × 5 = 40.

8 × 2 = 16.

Add the parts: 40 + 16 = 56.

So 8 × 7 = 56.

We can write it like this:

8 × 7 = 8 × 5 + 8 × 2.

QUESTIONS ABOUT THE STORY

How many rows did Samantha plant?

$A rectangle of unit squares, eight rows of seven, split into 5 and 2 columns$ 8 × 7 = 8 × 5 + 8 × ?

Samantha adds the two small products. 40 + 16 = ?

FILL IN THE MISSING NUMBER

$A rectangle of unit squares, six rows of eight, split into 5 and 3 columns$ 6 × 8 = 6 × 5 + 6 × ?

$A rectangle of unit squares, seven rows of six, split into 5 and 1 columns$ 7 × 6 = 7 × 5 + 7 × ?

$A rectangle of unit squares, nine rows of seven, split into 2 and 5 columns$ 9 × 7 = 9 × 2 + 9 × ?

$A rectangle of unit squares, four rows of nine, split into 5 and 4 columns$ 4 × 9 = 4 × 5 + ?

FIND THE PRODUCT

$A rectangle of unit squares, six rows of eight, split into 5 and 3 columns$ 6 × 8 = ?

$A rectangle of unit squares, seven rows of six, split into 5 and 1 columns$ 7 × 6 = ?

$A rectangle of unit squares, nine rows of seven, split into 2 and 5 columns$ 9 × 7 = ?

Samantha plants 8 rows of 9 acorns. How many acorns?

WHY THIS EXERCISE7 × 5 + 6 = 41, but 7 × 6 = 42.

Try it

Build an array of 4 rows of 7 with small things.
Lay a pencil across it to split each row into 5 and 2.
Find 4 × 5 and 4 × 2.
Add them to find 4 × 7.

On paper, draw 6 rows of 7 squares. Split them and write the break-apart.

What a clever trick! Tomorrow Freddy has a review game.

Grown-up checked my answers