That is very true. "Ideal" circumstances were the entire premises set forth when the question was posed in the beginning. There are other factors that can not be totally included. Those included power loss when the unloadinng auger is engaged, power increased as bin is emptied, inclination and soil conditions, the actual loading geometry on the unloading auger, and the items you mentioned. We could even question if 6.0 MPH is constant. The question was answered from a classical sense of the stated problem, maintaining all variables as constants.