To find what percent is the student's estimate off you use the next formula:
[tex]\frac{\lvert e-v\rvert}{v}\cdot100[/tex]e is the estimate volume
v is the real volume of the container
[tex]\begin{gathered} \frac{\lvert0.4-0.5\rvert}{0.5}\cdot100 \\ \\ \frac{\lvert-0.1\rvert}{0.5}\cdot100 \end{gathered}[/tex]The absolute value of a negative number is:
[tex]\lvert-a\rvert=a[/tex]Then:
[tex]\frac{0.1}{0.5}\cdot100=20[/tex]Then, The student's estimate is off by 20%.