What is the percent error of a length measurement of 0.229 cm if the correct value is 0.225 cm?

The percent error of this length measurement is 1.77%. 
Here is how you find this error.

First, you have to find
how far off your estimate was from the actual correct value.  In this case, we can see
that the difference was .004 centimeters.

Now, we need to
find out what this error is as a percent of the actual measurement.  That is a way of
showing how badly our estimate was incorrect.  We do this by dividing the error (.004)
by the correct answer (.225).  This gives us a result of
.01777.

To make this into a percentage, we multiply that by
100 and we get 1.77%.