Multiplication

Syllabus tag: Kenya CBC | Grade 6 Mathematics | Strand 1.0 Numbers | Sub-Strand 1.2 Multiplication (6 Lessons)

Lesson objectives

By the end of this topic, you will be able to:

• Multiply up to a 4-digit number by a 2-digit number
• Estimate products by rounding numbers to the nearest ten
• Make patterns involving multiplication of numbers not exceeding 1,000
• Appreciate the use of multiplication in real life

Multiplication

Multiplying is repeated adding. This topic multiplies larger numbers and checks the answers by estimating.

a) Long multiplication

Split the second number into its ones and its tens.

Multiply by the ones first, then by the tens, then add the two parts.

When you multiply by the tens, remember the zero. 1476 × 20 is 29 520, not 2 952.

Keep your columns straight. Most errors in long multiplication come from digits sliding out of line.

b) Estimating the product

An estimate is a quick rough answer. It tells you whether your careful answer is sensible.

Round each number to the nearest ten, then multiply.

The estimate is 44 400 and the exact answer is 41 328. They are close, so the exact answer looks right.

Suppose your exact answer were 4 132 or 413 280. The estimate would show at once that something had gone wrong.

Estimate before you calculate. Then you know what to expect.

c) Multiplication patterns

Multiplying by 10 adds one zero. Multiplying by 100 adds two.

So 34 × 10 is 340, and 34 × 100 is 3 400.

Look for patterns in a times table. In the 9 times table the digits of each answer add to 9. Look at 18, 27, 36 and 45.

Doubling and halving can make a sum easier. For 25 × 16, double 25 to get 50. Halve 16 to get 8. Then 50 × 8 is 400, the same answer.

d) Useful checks

Multiplying by an even number always gives an even answer.

Multiplying by 5 always ends in 0 or 5.

Multiplying by 0 always gives 0, however big the other number.

e) Where this is used

Working out the cost of many items at one price. Finding how many seats in a hall with equal rows. Calculating area. Working out total pay from a daily rate.

Words to know

• Product — the answer to a multiplication.
• Factor — a number being multiplied.
• Expanded form — a number written as the sum of its place values (e.g. 23 = 20 + 3).
• Estimate — a sensible approximate answer, found by rounding before calculating.

:::checkpoint Check yourself

1. Work out 2 314 × 32.
2. Estimate 1 987 × 41 by rounding to the nearest ten.
3. What happens to a number when you multiply it by 100?
4. Why should you estimate before calculating? :::

Bridge to practice

Try the exercises below — they mix direct multiplication, estimation, patterns, and real-life problems, the same four skills covered above.