Its to do with the method they used , the power output they want at 10 m of height is instantaneous - the method relies on the straight gradient of the graph to calculate the power output over 20 seconds (from 15s to 35s) so they just divide by 20 at the end to figure out the power at 10 m

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In the explanation, they took the time interval 15s to 35s to calculate the power, which equals work done/time. Work right here can be expressed by m*g*h and time = 35-15=20. 2000/20 =100W

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