In the given circuit the current flowing through the resisitance $20$ $\mathrm{ohms}$ is $0.3$ $\mathrm{ampere}$ while the ammeter reads $0.8$ $\mathrm{ampere}.$ What is the value of $R_1$? ................ $\mathrm{ohm}$
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Total current in ammeter is $0.8 \mathrm{A}$
Current through $20\, ohm$ resistor is $0.3 \mathrm{A},$ hence the voltage across it is
$V=0.3 \times 20=6 V$
Current through $15 \,ohm$ resistor is
$I=\frac{V}{R}=\frac{6}{15}=0.4 A$
Hence current through $\mathrm{R} 1$ is, $\mathrm{I}^{\prime}=0.8-(0.3+0.4)=0.1 \mathrm{A}$
Therefore,
$R 1=\frac{V}{I^{\prime}}=\frac{6}{0.1}=60 \Omega$
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