Transcript Multiplying and dividing decimals

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Decimal rules
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Multiplying with decimals
Performing multiplications is simpler with whole numbers than
it is with decimals. As a result, we often perform
an equivalent whole number calculation.
2.28 × 7 is equivalent to 228 × 7 ÷ 100.
228
× 7
15 96
15
1596 ÷ 100
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Do not forget to
carry across
when using the
column method.
= 15.96
Remember that dividing
by 100 means moving
the decimal point two
places to the left.
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Long multiplication
With larger numbers, we can also perform long multiplication.
What is 57.4 × 24?
Estimate: 60 × 25 = 1500
Equivalent calculation: 57.4 × 10 × 24 ÷ 10
= 574 × 24 ÷ 10
574
× 24
4 × 574 =
2296
20 × 574 = 11480
13776
Answer: 13776 ÷ 10 = 1377.6
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Long multiplication
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Short division by a decimal
We are dividing by a single digit which
means it is most efficient to use short
division to complete the calculation.
Calculate 57.23 ÷ 0.8 to 2 decimal places.
07 1 . 5 3 7
8 55712 .4 33060
We need to find the value of
this digit to see whether we
need to round up or down.
57.23 ÷ 0.8 = 71.54 (to 2 d.p.)
How would we complete a calculation when dividing
by a two-digit number?
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Dividing by two-digit numbers
How would you calculate 75.4 ÷ 3.1?
Estimate the answer first: 75 ÷ 3 = 25
The equivalent calculation is: 75.4 ÷ 3.1 = 754 ÷ 31
31
754
– 620
20 × 31
Our estimate will
134
help to give us an
– 124
4 × 31
accurate starting
10.0
point for the
– 9.3
0.3 × 31
calculation.
0.70
– 0.62
0.02 × 31
0.08
The answer is: 75.4 ÷ 3.1 = 24.32 R 0.08 = 24.3 to 1 d.p.
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Long division
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File storage
Rachel wants to buy a memory stick that she’s going to use to
store music she downloads.
She’s found three different types:
a 1 GB stick costing $6.99
a $12.79 stick with 2 GB
of memory
a 4 GB stick for $20.49.
If we know that there are 1024 MB in 1 GB, how much
would it cost to fill the three memory sticks?
Rachel buys the 4GB stick and uses 75% of the space.
How much has it cost her to buy and fill the stick?
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