Question

A lamp is 10% efficient.How much electrical energy must be supplied to the lamp each second if it produces 20 J of light energy per second?

Answer 1

If it produces 20J of light energy in a second, then that 20J is the 10% of the supply that becomes useful output.

20 J/s = 10% of Supply

20 J/s = (0.1) x (Supply)

Divide each side by 0.1:

Supply = (20 J/s) / (0.1)

Supply = 200 J/s  (200 watts)

========================

Here's something to think about:  What could you do to make the lamp more efficient ?  Answer:  Use it for a heater !

If you use it for a heater, then the HEAT is the 'useful' part, and the light is the part that you really don't care about.  Suddenly ... bada-boom ... the lamp is 90% efficient !