The work that the firefighter needs to do equals the force that he has to apply which is equal to his weight. So the firefighter's force times the distance over which that force is applied. In other words the height that he climbs. And there’s no horizontal forces in this question because we assume he’s going at constant speed horizontally. So there’s no net force that way. So it’s only this vertical force against gravity times the distance that he climbs. So in the sixth edition we have 65 kilograms times 9.8 newtons for every kilogram times 20 meters and that gives 1.27 times 10 to the 4 joules. In the fifth edition you have 75 kilograms here and the firefighter climbs 10.0 meters giving a fifth edition answer of ### times ## to the # joules.

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