Alight bulb converts electrical energy into light energy and also some thermal energy. the amount of electrical energy is 50 joules before the conversions.

If the amount of electrical energy is 50 Joules before the conversions, then it would be 50 Joules after the conversion.

According to law of conservation of energy, we cannot create or destroy energy so it remains constant

Hope this helps!

Independent variable is something that is changed in a experiment.