A windmill is taking advantage of strong

A windmill is taking advantage of strong air currents in order to produce electrical energy.  On a typical day the wind speed is around 20 mph and in that speed the windmill produces 800 kw/h (kilowatts per hour). On a stormy day a windmill produces 20% more energy.  How much kw/h can three windmills produce in two hours on a stormy day?

(a) 2880.
(b) 4860.
(c) 5780.
(d) 5760.
(e) 6380.

The best answer is D.
On a stormy day, a windmill will produce 20% more energy. 20% of 800 is 160, so each windmill will give out 960 kw/h. Three windmills will give 3 x 960 = 2880, which is answer (a), but we want two hours so the answer is 2880 x 2 = 5760.