This is actually a pursuit problem. The unit price can be understood as the distance, and the difference between them is 30. The electricity charge required for one hour can be understood as speed, which is100 * 0.5/1000 = 0.05 respectively.

40*0.5/ 1000=0.02

Then you can set the time to x.

(0.05-0.02)*X=30

X = 1000 hours

So the life of 1000 hours is the critical point. At this time, the electricity they need is

100* 1000/ 1000*0.5+2=52

40* 1000/ 1000*0.5+32=52

If it exceeds, the energy-saving lamp is cost-effective.