Step by step
The question
Question: what is the percent error, if the guess is 18 and the actual value is 20?
Percent error says how far off a guess is, as a percent of the real value.
1 Find the error
First: how far off is it?
The bigger one minus the smaller one.
20 minus 18 is 2.
The error is never negative: it is a distance.
2 Compare with the actual
Now we compare the error with the real value.
The error, 2, goes on top.
The actual value, 20, goes at the bottom.
Careful!
We divide by the actual value, 20.
Not by the guess.
The real value is what we compare with.
3 Write it as a percent
2 and 20 can both be divided by 2.
That keeps the numbers small: 1/10.
Percent means out of 100.
10 times 10 is 100.
So we multiply the top and the bottom by 10.
1 times 10 is 10.
10 out of 100 is 10%.
So the percent error is 10%.
The guess was 10% too low.
4 Check
Let's check: 10% of the actual value should give the error.
10% is 0.1.
0.1 times 20 is 2.
That is the error, 2.
So the answer is right!
Let's look back.
First: find the error.
Then: compare with the actual.
Then: write it as a percent.
Last: check.