Physics初中電路 http://upload.lsforum.net/users/public/u2711^g72.png?

(a) I1 = Vs/R1 = 12/75 A = 0.16 A

(b) When S2 is open, no current flow through R4. Hence, I3 = 12/(20+150) A
= 0.0706 A
[ Thanks for DL081797 for pointing out my mistake in my original calculation. It has been corrected now ].

(c) Equivalent resistance of R4 and R5 in parallel
= 100 x 150/(100 + 150) ohms = 60 ohms
Hence, voltage across R4 = 12 x [60/(20 + 60)] ohms = 9 volts
Current I4 = 9/100 A = 0.09 A

(d) Equivalent resistance of R3 in series with R4 and R5 in parallel
= (20 + 60) = 80 ohms
Hence, equivalent resistance of the circuit
= 1/[(1/75) + (1/50) + (1/80)] ohms = 21.81 ohms
Therefore, Is = 12/21.81 A = 0.55 A