The actual value = 25.7 ml
The error = 15.6%
This means that the value of error = 0.156 * 25.7 = 4.0092 ml
Now, the error percentage found means that the student got the value either greater than the actual one with the value of error or less than the actual one with the value of error.
This means that the two possible readings are:
either : 25.7 + 4.0092 = 29.7092 ml
or : 25.7 - 4.0092 = 21.6908 ml
