SOLUTION: when paying her telephone bill lucy paid $45 instead of $40 ,what is percentage error?

There are two ways to do it.

First way:

We find the ratio of the mistaken amount to the correct amount:



So the mistaken amount paid was 12.5% more than 100% of the correct 
amount.  I.e., subtract 100%

Answer: 12.5%

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Second way:

We subtract the right amount from the wrong amount.

$45 - $40 = $5

Then we find what percent of the right amount $5 is

.

Both ways are correct.  Take your pick.

Edwin