How do I find the reactive force of a person pulling a vertical rope?

Jun 2019
R is the force (pair) between the person and the platform. There are three forces on the person: The rope pulling up, his weight pulling down, and the platform pushing up (R). If the platform pushes up on the person with a force R, the person pushes down on the platform with force R (Newton's 3rd Law). The 80 N force acts between the person and the Earth (or whatever the local source of gravity is); it doesn't affect the platform directly.

As for the units, I suspect this was originally a problem in pounds, and someone changed the units without changing the numbers. Obviously a non-engineer. ;)