The radius $(r)$,length $(l)$,and resistance $(R)$ of a metal wire were measured in the laboratory as:
$r = (0.35 \pm 0.05) \text{ cm}$
$R = (100 \pm 10) \text{ } \Omega$
$l = (15 \pm 0.2) \text{ cm}$
The percentage error in the resistivity of the material of the wire is: (in $\%$)

Two resistances are measured in $\Omega$ and are given as $R_1 = 3 \Omega \pm 1\%$ and $R_2 = 6 \Omega \pm 2\%$. When they are connected in parallel,the percentage error in equivalent resistance is .......... $\%$

Why is the error in a measurement expressed with both positive and negative signs?

Zero error of an instrument introduces

The density $\rho$ of a uniform cylinder is determined by measuring its mass $m$, length $l$ and diameter $d$. The measured values of $m, l$ and $d$ are $97.42 \pm 0.02 \text{ g}$, $8.35 \pm 0.05 \text{ mm}$ and $20.20 \pm 0.02 \text{ mm}$, respectively. The calculated percentage fractional error in $\rho$ 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 $\%$?