Question

if the electric current in a lamp decreases 5% then the power output decreases by

Answer 1

If the current drops the filament temperature drops and consequently resistance, cause it depends on temperature in a bulb with metal filament (tungsten typically). This only could happen if voltage drops too!, and don’t use Ohms law!

You need to resolve these:

k⋅tc⋅dTdt=I2⋅R−k⋅Tk⋅tc⋅dTdt=I2⋅R−k⋅T

R=R0⋅(1+α⋅T)R=R0⋅(1+α⋅T)

Or a more detailed heat transfer model.

Or look for a table of typical values for a light bulb in internet,… for a current drop to 98% voltage drops to 96% aprox, so power drops 6%