Physics

I have very little knowledge when it comes to physics, but don't you need a location to determine his mass elsewhere? As in, a particular planet, a gravity simulation chamber, so on?

W = mg
weight = mass*gravitational field strength

g ~ 10
its closer to 9.8 or summat but if ure studying physics at GCSE or below you'll be told to use 10

W = 80 * 10
= 800N