Step by step
The question
Question: find the radius r of a circle with circumference 31.4.
1 Write the rule
The circumference is 2 times π times the radius: C = 2 × π × r.
π is a special number for circles.
It is about 3.14.
Now put the numbers in: C = 31.4 and 3.14 for π.
Careful!
31.4 divided by 3.14 is the diameter.
The radius is half of that.
2 times 3.14 is 6.28.
So 6.28 × r = 31.4.
2 Remove the × 6.28
We want r by itself.
The times 6.28 is in the way.
To remove times 6.28, we divide by 6.28.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
6.28 divided by 6.28 is 1.
So just r is left.
Check the division: 6.28 times 5 is 31.4.
31.4 divided by 6.28 is 5.
So the radius is 5.
3 Check
Let's check: put r = 5 back in the rule.
We get 31.4 again.
So the radius is right!
Let's look back.
First: write the rule.
Then: remove the × 6.28.
Last: check.