###Human: The price of electricity went up by 25%.  John's old computer used 800 watts and his new computer uses 50% more.  If the old price of electricity was 12 cents per kilowatt-hour, how much does his computer cost, in dollars, to run for 50 hours?, ### Assistant: His new computer uses 800*.5=400 watts more than his old computer
So it users 800+400=1200 watts
That is 1200/1000=1.2 kw
Electricity cost 12*.25=3 cents more than it used to
So it now cost 12+3=15 cents per kwh
He used 1.2*50=60 kwh
So he paid 15*60=900 cents per kwh
So he paid 900/100=$9 
 So the answer is :  9