The efficiency of the light bulb is 8.9%. This means that only 8.9% of the power of the lightbulb becomes useful power, and it corresponds to:
[tex]P_u = \frac{8.9}{100} P = \frac{8.9}{100} (15 W)=1.34 W[/tex]
The rest of the power instead is wasted as heat, and this amount of power wasted is given by the difference between the total power and the useful power:
[tex]P_w = P-P_u = 15 W - 1.34 W=13.66 W[/tex]
So, the power wasted as heat is 13.66 W.
