^{3}-kg aircraft takes 12.5 min to achieve its cruising altitude of 10.0 km and cruising speed of 850 km/h. If the plane's engines deliver, on the average, 1500 hp of power during this time, what is the efficiency of the aircraft engines?

I initially believed the power out would be: (Kinetic energy + Potential energy)/Time

And efficiency would be 1500hp/power out. But after my calculations, I got 14.6% efficiency, when it should be 48.7% efficiency.

What did I do wrong?