Write the data down:
i = 2.5 A
V = 120 V
t = 1.2 h
First we must calculate electric power of the computer:
P = i x V
P = 2.5 x 120
P = 300 W
Now transform the time in minutes and then in seconds:
1.2 h = 60 min x 1.2 h = 72 minutes
72 minutes x 60 s = 4,320 s
So the energy cost will be:
E = P x t
E = 300 x (4,320)
E = 1,296,000 J
or if we write using scientific notation we get:
