Question

In the circuit shown, the battery is ideal and has voltage 21V. Reading of $\left(A_1\right)$ ammeter is 0.42A. When another identical ammeter is also connected across R reading of $A_1$ becomes 0.5A, then resistance of ammeter in ohms is

Answer 1

$21=0.42(R+R_A)\dots \dots \dots \left(1\right)$ and $21=0.5[\frac{R{R}_A}{R+R_A}+R_A]\dots \dots \dots \left(2\right)$
Solving (1) and (2) we get $R=\frac{2}{3}{R}_A$
Again by eq. (1) we get $R_A=30\Omega$