Well first of all, I think the students may have been correct. If they didn't use distilled water, and if it wasn't exactly at standard temperature, then the mass of 25.0 mL could very well be 25.4 grams. We don't know that there was any 'error' in their measurement at all. But the question says there was, so we'll do the math:
The 'error' was (25.4 - 25.0) = +0.4 gram
As a fraction of the 'real' value, the error was
+0.4 / 25.0 = +0.016 .
To change a decimal to a percent, move the decimal point two places that way ===> .
+ 0.016 = +1.6 % .
Their measurement was 1.6% too high.
Let's not call it an 'error'. Let's just call it a 'discrepancy' between the measured value and the 'accepted' value. OK ?
