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?

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