Question

Clea estimates that a glass contains 250.55 mL of water. The actual amount of water in the glass is 279.48 mL. To the nearest tenth of a percent, what is the percent error in Clea's estimate?

Answer 1

The only thing you need to solve this task is to know the percent error formula:

 (theoretical value-experimental value): theoretical value)) * 100
Your theoretical value is 279, as it is the value that you know, and the experimental -250 is your calculated value.
279-250/279
29/250
0.116*100
11.6%
Then you can recheck everything: 
250*0.116=29
250+29=279