Astudent determined in the laboratory that the percent by mass of water in cus04 5h20 is 38.0% if the accepted value is 40% what is the percent of error

Percent error is [(abs(experimental - actual))/(actual)] * 100
(38.0 - 40.0 ) / 40.0 = 2.0 / 40 =  0.05 
0.05 * 100 = 5
since there are 2 sigfigs, the answer is 5.0 %

I made the 2 positive since the absolute value of 38.0 - 40.0 was taken
There are only 2 sigfigs because of the subtraction of 38.0 - 40.0 gives 2.0, only 2 sigfigs