How many lamps each of 50 W and 100 V can be connected in parallel across a 120 V battery of internal resistance 10 Q, so that each glows to full Power?

The resistance of each lamp is
R = V/P = 100/50 = 200Ω.
Let n be the number of lamps. The total resistance in parallel is:
1/Req = n/200
Including battery resistance:
Rtotal = 200/n + 10
For full power:
Vlamp = 100V ⇒ I = 120/200/n + 10
Solving for n, we get 4. Answer:(b) 4.