Explanation:
the first step in my opinion would be find the norm(s) in this scenario.
100 pounds is a good norming weight.
the speed of these 100 pounds with a 10 HP motor is
50ft/30sec = 5/3 ft/s
so, then let's ask next what size of motor is needed to lift the same 100 pounds 40 ft in 40 seconds
40ft/40sec = 1 ft/s
to get the same speed as in the first case the motor need to be weaker (to move slower). and that goes by the same factor as the ratio between the 2 speeds :
5/3×f = 1
f = 3/5
10 HP × f = 10 × 3/5 = 30/5 = 6 HP
so, it takes a 6 HP motor to lift these same 100 pounds 40 ft in 40 seconds.
and now, how strong must the motor be to lift 800 pounds (= 8 times the original 100 pounds) with the same speed ?
well, 8 times
8×6 = 48 HP
so, it would take a 48 horsepower motor to lift 800 pounds 40 ft in 40 seconds.
the equation would then be
HP = p/100 × 6 = 6p/100 = 3p/50
HP = horsepower
p = pounds
the rest I cannot really answer, as I don't know your class and it's contents.
I hope I covered the topics of your class with that solution.