Can error be completely eliminated?

The errors in the measurement of mass and length of a cube are $1.5 \%$ and $2.5 \%$ respectively. The percentage error in the measurement of the density of the cube is: (in $\%$)

The period of oscillation of a simple pendulum is $T = 2 \pi \sqrt{L / g}$. The measured value of $L$ is $20.0 \; cm$ known to $1 \; mm$ accuracy,and the time for $100$ oscillations of the pendulum is found to be $90 \; s$ using a wrist watch of $1 \; s$ resolution. What is the accuracy in the determination of $g$ in $\%$?

The percentage error in the measurement of mass and speed are $3\%$ and $2\%$ respectively. Then,the percentage error in kinetic energy will be .......... $\%$

Students $A, B$ and $C$ measure the length of a room using a $25 \,m$ long measuring tape of least count $0.5 \,cm$,a meter-scale of least count $0.1 \,cm$,and a foot-scale of least count $0.05 \,cm$,respectively. If the actual length of the room is $9.5 \,m$,which of the following students will report the lowest relative error in the measured length?

Calculate the mean percentage error in five observations: $80.0, 80.5, 81.0, 81.5, 82.0$. (in $\%$)