Question

there are three sources of resistance in a parallel ciruit. Two of them are rated at 20ohms, the other at ohms. What is the curuit's total resistance? A)20 ohms B)12 ohms C)8 ohms D)5 ohms

Answer 1

letter B

Answer 2

You haven't stated the third resistance, but both A) and B) are not possible for any value in parallel with two 20 ohm resistors. The only possible answers are C) and D).

using 1/R(total) = 1/R +1/R +1/R

1/20 +1/20 =0.1

if the unstated value =x

A)

so 0.1 +1/x = 1/20 = 0.05
which requires a -ve value for x

B)

so 0.1 + 1/x = 1/12 = 0.0833
which requires a -ve value for x

C)

so 0.1 + 1/x = 1/8 = 0.125
which requires a value for x=40 ohms

D)

so 0.1 + 1/x = 1/5 = 0.2
which requires a value for x=10 ohms