Decimal Calculations 5
Multiplication with Decimals

We now extend our multiplication to decimals.

Worked Examples

1

You know that 35 × 19 = 665.

Deduce the value of

(a)

3.5 × 19

Show me
3.5 × 19 = × 19
= 35 ×
=
= 66.5
(b)

3.5 × 1.9

Show me
3.5 × 1.9 = ×
= 35 ×
=
= 6.65
(c)

350 × 1.9

Show me
350 × 1.9 = (35 × 10) ×
= 35 × 10 ×
= 665
(d)

350 × 190

Show me
350 × 190 = (35 × 10) × (19 × 10)
= (35 × 19) × (10 × 10)
= 665 × 100
= 66 500

Of course, in practice you do not need to write out the calculations in full like this, but simply write down the answers.

Exercises

Find:

(a)
3 × 0.8
(b)
5 × 0.7
(c)
3 × 2.6
(d)
9 × 1.2
(e)
6 × 1.5
(f)
8 × 7.9
(g)
2.1 × 3.2
(h)
5.6 × 7.2
(i)
8.4 × 2.1
(j)
9.2 × 1.8
(k)
1.2 × 6.2
(l)
15 × 7.3
(m)
22 × 9.4
(n)
62 × 7.1
(o)
74 × 5.3

Work out the following, using a quick method if possible.

(a)
6 × 10
(b)
0.7 × 10
(c)
12.2 × 100
(d)
112 × 10
(e)
2 × 3.2 × 5
(f)
2 × 62 × 50
(g)
1.47 × 1000
(h)
18.41 × 10
(i)
365 × 100
(j)
200 × 7200 × 5

Find:

(a)
2.47 × 1.6
(b)
3.25 × 11.1
(c)
3.42 × 6.19
(d)
7.24 × 5.16
(e)
8.21 × 15.1
(f)
32.1 × 0.47