Cogito
Grade 3 Math · Chapter 3 · Lesson 4
Multiplying by 8 and 9
Doubling again, and a pattern worth spotting.
10 problems · about 14 minutes · 3.OA.C.7, 3.OA.B.5
What this lesson teaches
The child recalls the 8 and 9 times tables, using repeated doubling and the tens-minus-one pattern.
- Eights are double fours.
- Nines are tens with one group taken away.
- The digits of every nine answer add to 9.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — Multiplying by 7: 8 × 7 = ?
Answer 56
Why 8 × 7 = 56.
Review — Multiplying by 6: 9 × 6 = ?
Answer 54
Why 9 × 6 = 54.
Warm Up
Straightforward practice. Get the method working first.
4 problems7 × 9 = ?
Answer 63
Why 7 × 9 = 63, and 6 + 3 = 9.
In the nine times table, what do the digits of each answer add to?
Answer 9
Why The digits always add to 9.
4 × 8 = ?
Answer 32
Why Double 16.
8 × 8 = ?
Answer 64
Why Double 32.
Build It Up
The same ideas with more to keep track of.
2 problems6 × 9 = ?
Answer 54
Why 60 − 6.
72 ÷ 8 = ?
Answer 9
Why What times 8 makes 72?
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Spiders: 6 spiders each have 8 legs. How many legs?
Answer 48 legs
Why 6 × 8 = 48.
The Rows: 9 rows of 9 chairs. How many chairs?
Answer 81 chairs
Why 9 × 9 = 81, and 8 + 1 = 9.