An energy-efficient water heater draws 4 amps in a 220-volt circuit. It costs $75 more that a standard water heater that draws 18 amps in a standard 115-volt circuit. If electricity costs 10 cents per kilowatt-hour, how long would you have to run the efficient water heater to recoup the difference in price? Show your calculations.

The answer to the question above is "630.25 hours" which is the time to recoup the difference in the water heater cost. This problem can be solved using a simple algebra equation which stated as (18*115*0.1/1000) x - (4*220*0.1/1000) x = $75. In this formula, x represents the time to recoup the difference in the water heater cost (Calculation: 0.207x-0.088x=75 - - > 0.119x = 75 - - > x = 630.25).