Step by step
The question
Question: A rectangle is 12 cm by 7 cm, both to the nearest cm.
Find the upper bound of its area.
1 Bounds of each number
The length is 12 cm, to the nearest cm.
Half a step is 0.5.
So it is from 11.5 up to 12.5.
That is 12, plus or minus 0.5.
The width is 7 cm, to the nearest cm.
Half a step is 0.5.
So it is from 6.5 up to 7.5.
That is 7, plus or minus 0.5.
2 Choose the bounds
To make the area as big as it can be, both numbers must be as big as they can be.
So we use 12.5 and 7.5.
Careful!
We do not change the answer by half a step.
We work it out again with the biggest numbers.
3 Work it out
With a calculator: 12.5 times 7.5 is 93.75 cm².
So the upper bound of the area is 93.75 cm².
4 Check
Let's check.
With the rounded numbers we get 84.
The upper bound, 93.75, is more than that, as it should be.
So the answer is right!
Let's look back.
First: bounds of each number.
Then: choose the bounds.
Then: work it out.
Last: check.