The least count of a stop watch is $0.2$ second. The time of $20$ oscillations of a pendulum is measured to be $25$ seconds. The percentage error in the time period is

The mass of a ball is $1.76 \mathrm{kg}$. The mass of $25$ balls is

You are given two unknown resistors $X$ and $Y$ These resistances are to be determined, using an ammeter of $R_A=0.5 \mathrm{\Omega }$ and a voltmeter of $R_v=20 \mathrm{k\Omega }$. It is known that $X$ is in range of a few ohms and $Y$ is in the range of several kilo ohm's. Which circuit is preferable to measure $X$ and $Y$.

$\begin{array}{r}\text{ Resistor }\\ X\\ \mathrm{y}\end{array}\begin{array}{r}\text{ Circuit }\\ a\\ \mathrm{b}\end{array}$