Question

Three resistances of 12, 16 and 20 ohm are connected in parallel. What
resistances must be connected in series this combination to give a total resistance
of 25 ohm?
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Answer 1

Total resistance in parallel is Rp

So (1/Rp) = (1/12)+(1/16)+(1/20)

Rp = ?

impedance is 25

25 = Rp + Rs

put the value of Rp you'll get the answer.