Number-Level 3
Rounding Up and Down
You need
a classmate
Activity
1.
Colin is investigating rounding. He discovers that 47, which is closer to
50 than to 40, is usually rounded to 50.
0
10
20
30
40
47 50
60
He also discovers that numbers ending in 5 are usually rounded up.
Round these numbers to the nearest 10:
a. 34
2.
b. 66
c. 91
d. 108
e. 275
f. 999
Colin then looks at rounding a decimal to the nearest whole number.
He decides that 2.3 is closer to 2 than to 3, so 2.3 rounded to the
nearest whole number is 2.
0
1
2
2•3
3
4
5
6
Round these decimals to the nearest whole number:
a. 1.6
3.
b. 2.9
c. 4.3
d. 7.5
e. 18.4
f. 276.8
Discuss with a classmate some situations where rounding up
may be a more sensible option than rounding down.
Investigation
Hirini is going to the timber yard to buy timber for
his fence.
He can buy the timber in lengths of 3 metres and 5 metres.
Three metre lengths cost $13.80, and 5 metre lengths
cost $23.
Hirini needs to cut his timber into these lengths:
•
four 0.8 metre lengths
•
two 1 metre lengths
•
two 1.2 metre lengths
•
four 1.8 metre lengths
•
six 2.4 metre lengths
•
two 3.2 metre lengths.
He wants his timber bill to be as cheap as possible.
How many 3 metre lengths and how many 5 metre
lengths should Hirini buy?
Making sensible estimates
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