Question

If $i=0.25$ amp. in the fig the value of R is :-

• A. $48\Omega$
• B. $12\Omega$
• C. $120\Omega$
• D. $42\Omega$

Answer 1

The correct option is D $42\Omega$

The equivalent resistance of circuit is $R_{eq}=R+60||\left(20||10\right)=R+60||\left(\frac{20\times 10}{20+10}\right)=R+60||\left(20/3\right)=R+\frac{60\times 20/3}{60+20/3}=R+\left(1200/200\right)=R+6$
Now current , $i=\frac{12}{R+6}=0.25$
or $R+6=\frac{12}{0.25}=48$
$\therefore R=48-6=42\Omega$