Cogito
Grade 3 Math · Chapter 3 · Lesson 6
Multiplying by Multiples of Ten
Multiply the digits, then count the tens.
10 problems · about 13 minutes · 3.NBT.A.3
What this lesson teaches
The child multiplies a one-digit number by a multiple of ten from 10 to 90.
- 4 × 30 is 4 groups of 3 tens.
- Multiply the digits, then say the answer in tens.
- The zero appears because the unit is a ten.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
2 problemsReview — The Associative Property: 2 × 5 × 6 = ?
Answer 60
Why Grouping the 2 and 5 first makes it 10 × 6.
Review — Multiplying by 8 and 9: 7 × 9 = ?
Answer 63
Why 7 × 9 = 63, and 6 + 3 = 9.
Warm Up
Straightforward practice. Get the method working first.
4 problems7 × 40 = ?
Answer 280
Why 7 × 4 = 28, so 7 × 40 = 280.
Why does 4 × 30 end in a zero?
Answer Because you counted tens, not ones.
Why The answer is a number of tens, which is why it ends in 0.
3 × 40 = ?
Answer 120
Why 12 tens.
8 × 50 = ?
Answer 400
Why 40 tens.
Build It Up
The same ideas with more to keep track of.
2 problems9 × 80 = ?
Answer 720
Why 72 tens.
6 × 60 = ?
Answer 360
Why 36 tens.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Boxes: 5 boxes each hold 20 pencils. How many pencils?
Answer 100 pencils
Why 10 tens is 100.
The Seats: 7 rows of 30 seats. How many seats?
Answer 210 seats
Why 21 tens is 210.