Yep, just confirming what Dave said - I was typing my reply at the same time so I'll post it anyway and it may help clarify the problem for you.
You start out knowing the European voltage and wattage - from that can be determined the current, and from that can be determined the resistance.
Calculate the current first, using I = P / V, where I is the current (amps), P is the power (watts), and V is volts, so:
I = P / V
I = 100 / 220
I = 0.454545 Amps
You can now calculate the filament resistance, using R = V / I, where R is resistance, so:
R = V / I
R = 220 / 0.454545
R = 484.0 ohms
So how many watts will the bulb dissipate with 120 volts applied? Well firstly, calculate the current, using I = V / R, so:
I = V / R
I = 120 / 484.0
I = 0.248 amps
You can now calculate the wattage, using P = V * A
P = V * A
P = 120 * 0.248
P = 29.76 watts
And that's the answer.
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Once you're familiar with the relationships between voltage, current, resistance, and power, you can skip steps and tackle the problem thus:
R = V * V / P
R = 220 * 220 / 100
R = 484.0 ohms
then:
P = V * V / R
P = 120 * 120 / 484.0
P = 29.75 watts
The discrepancy of 0.01 watts between the two answers is because I only used six decimal places for the current, at step one in the first calculation, so retain as many decimal places as possible throughout, to achieve greater accuracy.