Question

What resistance must be connected to a $15\Omega$ resistance to provide an effective resistance of $6\Omega ?$

Answer 1

Let $x\Omega$ is connected in parallel to $15\Omega$ to provide an effective resistance of $6\Omega$

$⟹\frac{15x}{15+x}=6$

$⟹15x=6x+90$

$⟹9x=90⟹x=10$

Required resistance is $10\Omega$