Chapter 7 Decimals Worksheet - New Century Maths Page 19
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383 Given that 1.7 × 1.2 = 2.04, use estimates to find:
a 1.7 × 12
b 17 × 12
c 0.17 × 1.2
d 120 × 170
e 0.12 × 17
f 17 000 × 0.12
4 Given that 7.2 × 3.4 = 24.48, use estimates to find:
a 7.2 × 34
b 72 × 3.4
c 0.72 × 3.4
d 72 × 34
e 7.2 × 0.34
f 720 × 340
5 Given that 1.26 × 6 = 7.56, use estimates to find:
a 12.6 × 6
b 126 × 6
c 1.26 × 0.6
d 0.126 × 6
e 0.126 × 0.6
f 126 000 × 600
6 Use the fact that 0.3 × 0.24 = 0.072 to find:
a 3 × 0.24
b 0.3 × 2.4
c 0.3 × 24
d 30 × 0.24
e 300 × 2.4
f 0.03 × 0.24
Working mathematically
Reflecting and reasoning: Decimal places in multiplication answers
1 What happens when you multiply by a number less than one? As a group activity,
consider the product of 12 × 0.8.
a Is the answer more or less than 12? Why?
b Estimate the answer to 12 × 0.8.
c How many decimal places have 12 and 0.8?
d Use a calculator to evaluate 12 × 0.8. How many decimal places has the answer?
2 a Is the answer to 0.7 × 0.3 more or less than 0.7? Why?
b Estimate the answer to 0.7 × 0.3.
c How many decimal places have 0.7 and 0.3?
d Use a calculator to evaluate 0.7 × 0.3. How many decimal places has the answer?
3 a Is the answer to 2.5 × 4.1 more or less than 2.5? Why?
b Estimate the answer to 2.5 × 4.1.
c How many decimal places have 2.5 and 4.1?
d Use a calculator to evaluate 2.5 × 4.1. How many decimal places has the answer?
e What is the relationship between the number of decimal places in the question
and the number of decimal places in the answer?
4 a If 82 × 6 = 492, what do you think is the answer to 82 × 0.6? Why?
b If 4 × 17 = 68, what do you think is the answer to 0.4 × 1.7? Why?
c If 367 × 51 = 18 717, what do you think is the answer to 3.67 × 5.1? Why?
d What is the answer to 0.5 × 0.9?
When multiplying decimals, the number of decimal places in the answer
equals the total number of decimal places in the question.
5 Make up a question about multiplying decimals. Swap questions with all members
of the group. If you disagree about the correct answers, check with your teacher.
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NEW CENTURY MATHS 7
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