Question

If two resistors of 25 Ω and 15 Ω are joined together in series and then placed in parallel with a 40 Ω resistor, the effective resistance of the combination is : 

(a) 0.1 Ω
(b) 10 Ω
(c) 20 Ω
(d)40 Ω

Answer 1

 (c) 20 Ω
When 25 Ω and 15 Ω are connected in series, then:
Total resistance, R = 25 + 15 = 40 Ώ
This 40 Î is connected in parallel with the 40 Ώ resistor.
Therefore, the net resistance = 40 / 2 = 20 Î