Page 104 - Math Smart - 7

O Mitch needs 60.24 kg of potting soil to fill all his plant pots. He can only
buy potting soil in 5 kg bags.
Mitch uses rounding to estimate the amount of potting soil he needs to
the nearest kilogram.
60^5= 12
Mitch calculates that he needs 12 bags of potting soil.
a) Does Mitch have enough potting soil to fill his plant pots?
Explain your answer.
b) What should Mitch do?
Mitch should round the amount of soil to kg, instead of
rounding kg.
c) Calculate the number of bags of potting soil Mitch should buy.
d) How many kilograms potting soil will Mitch have left over?

In this investigation, we use estimation to make the numbers more manageable
and to get a sensible answer.

Check My
Understanding

Use your calculator to find the answers:
2.85 X 8.23
a) 5.16 X 3.81 b) 19.76^2.8
5.32 + 3.74
Another way of checking
© Show how you use estimation to check your calculations for Question 1,
your answer Is to do the
© Use estimation to decide quickly which is the correct answer for each
calculation a second time
calculation.
on the calculator. The
problem with this is that a) 14.1x2.3 b) 19.7 + 4.8 c) 11.3 + 13.9
you could repeat the
A. 3.243 A. 4.104 A. 2.52
mistake a second time.
B. 32.43 B. 41.04 B. 25.2
This is why it is useful to
use estimation to check C 324.3 C. 410.4 C. 252
your answer. D. 3243 D. 4104 D. 2520
O Harry writes down the following equation:
14.62 X 4 = 584.8
a) Use estimation to explain why Harry is not correct.

b) Calculate the correct answer and then describe the mistake that
Harry made.

UNIT 4 I Decimals