Step by step
The question
Question: A rectangle is 12 cm by 7 cm, both to the nearest cm.
Find the lower bound of its perimeter.
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.
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.
2 Choose the bounds
To make the perimeter as small as it can be, both numbers must be as small as they can be.
So we use 11.5 and 6.5.
Careful!
We do not change the answer by half a step.
We work it out again with the smallest numbers.
3 Work it out
That is 36 cm.
11.5 plus 6.5 is 18.
Two times that is 36.
So the lower bound of the perimeter is 36 cm.
4 Check
Let's check.
With the rounded numbers we get 38.
The lower bound, 36, is less 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.