You estimate that a jar contains 68 marbles. the actual number of marbles is 60. find the percent error. round your answer to the nearest tenth.

So the problem is to find out how much is 68 in percent if 60 is the 100%, we can use a rule of three, simple, direct proportion to do it:
60  >  100%
68 > x
x = (68)(100)/60
x = 113.3
therefore 68 is the 113.3% of 60, which means that the error is of:
113.3 - 100 = 13.3%

A