Zero error in an instrument introduces

What is error in measurement? What is mistake in measurement?

In an experiment,four quantities $p, q, r$,and $s$ are measured with percentage errors of $3 \%, 2 \%, 3 \%$,and $1 \%$ respectively. Quantity $A$ is calculated as follows: $A = \frac{pq^2}{r^2 s^4}$. The percentage error in $A$ is: (in $\%$)

The period of oscillation of a simple pendulum is given by $T = 2\pi \sqrt{\frac{l}{g}}$,where $l$ is about $100 \, cm$ and is known to have $1 \, mm$ accuracy. The period is about $2 \, s$. The time of $100$ oscillations is measured by a stopwatch of least count $0.1 \, s$. The percentage error in $g$ is ......... $\%$

$A$ physical quantity $S$ is related to four observables $a, b, c$,and $d$ as $S = \frac{\sqrt{a} b}{c^3 d^4}$. If the percentage errors of measurement in $a, b, c$,and $d$ are $2 \%$,$1 \%$,$1 \%$,and $1 \%$ respectively,then the percentage error in the quantity $S$ is: (in $\%$)

$A$ force $F$ is applied on a square area of side $L$. If the percentage error in the measurement of $L$ is $2 \%$ and that in $F$ is $4 \%$,what is the maximum percentage error in pressure (in $\%$)?