The resistance of the lamp is apparently 50V/2A = 25 ohms.
When the circuit is fed with more than 50V, we want to add another resistor in series with the 25-ohm lamp so that the current through the combination will be 2A.
In order for 200V to cause 2A of current, the total resistance must be 200V/2A = 100 ohms.
The lamp provides 25 ohms, so we want to add another 75 ohms in series with the lamp. Then the total resistance of the circuit is (75 + 25) = 100 ohms, and the current is 200V/100 ohms = 2 Amps.
The power delivered by the 200V mains is (200V) x (2A) = 400 watts.
The lamp dissipates ( I² · R ) = (2² · 25 ohms) = 100 watts.
The extra resistor dissipates ( I² · R) = (2² · 75 ohms) = 300 watts.
Together, they add up to the 400 watts delivered by the mains.
CAUTION: 300 watts is an awful lot of power for a resistor to dissipate ! Those little striped jobbies can't do it. It has to be a special 'power resistor'. 300 watts is even an unusually big power resistor. If this story actually happened, it would be cheaper, easier, and safer to get three more of the same kind of lamp, and connect THOSE in series for 100 ohms. Then at least the power would all be going to provide some light, and not just wasted to heat the room with a big moose resistor that's too hot to touch.
The refraction of light at the surface of water makes ponds and swimming pools appear shallower than they really are. A 1m deep pond would only appear to be 0.75 m deep when viewed from directly above. When light emerges from glass or water into air it speeds up again.
When the plastic ruler is rubbed, friction opposes the motion and causes the transfer of electron from one surface to another such that plastic becomes negatively charged. When ruler is brought nearer to the paper, it induces the positive charge in the piece of paper.
