need help solving this problem!

[Guest]

The 100W light bulb (cost .60 dollars) is not going to be used anymore and is being replaced with the 20W (cost 3 dollars) low energy light bulb.
If electricity is charged at $0.15 per kWh, for how many hours must the low energy bulb be used in order for the lower cost of running it to exactly compensate for its higher initial cost?

A. 0.25
B 250.00
C 200.00
D 160.00
E 720.00

answer: D

why?

On first appearance you would think:
0.60 + 0.1 * 0.15 * t = 3.00 + 0.02 * 0.15 * t
t = 200

But that's the wrong answer. That's one of the trap choices.

How do you get to 160?

What am I misunderstanding or missing?

[Guest]

Perhaps you are missing the difference between "Watts" and "kilowatts". In t hours a 100 Watt bulb will use 100t Watt hours= 100t/1000= 0.1t kilowatt hours of electricity. Similarly a 60 Watt bulb will use 60t Watt hours= 60t/1000= 0.06 kilowatt hours or electricity.