Sounds like an energy conservation problem to me. All of his gravitational potential energy can be assumed to be stored in the bungie at the bottom of his jump. You should be able to use the elastic potential equation 1/2kx^2 (where x is the stretch in the bungie) and equate that with the gravitational potential mgh
On a related note, are you sure that 25m is the correct free fall distance? I see where you get 20m (the rope) and 3m (the unstretched bungie) and 1m (the CM above the starting point) but where is the other 1m from?
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