Question

when two resistors are connected in series the total equivalent is 90 ohm. when in parallel the equivalent is 20 ohm what is the value of each resistor.

Answer 1

let the resistors be s and q
$s + q = 90 \: \: - (1)\\ \frac{1}{20} = \frac{1}{s} + \frac{1}{q} \\ 20 = \frac{sq}{s + q} \\ 20 = \frac{sq}{90} \\ sq = 1800 \: \: - (2)$
by solving (1) and (2)
s = 60 ohms
q = 30 ohms