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
