Question

If two resistors of $25\Omega$ and $15\Omega$ are joined together in series and then placed in parallel with a $40\Omega$ resistor, the effective resistance of the combinations is:
(a) 0.1 $\Omega$ (b) 10 $\Omega$ (c) 20 $\Omega$ (d) 40 $\Omega$

Answer 1

Two resistors of 25 $\Omega$ and 15 $\Omega$ are joined together in series.

So, R=25+15 = 40 $\Omega$

Now, this combination is placed in parallel with a 40 $\Omega$ resistor.

So, $\frac{1}{R_{eq}}=\frac{1}{40}+\frac{1}{40}$

$\Rightarrow R_{eq}=20\Omega$

Thus, the effective resistance of the combination is 20 $\Omega$.