Are you sure it's 81.5 N on the surface of the moon?
#1. Anyway, what we'll do is first get the mass of the man. Note that mass is constant, and in physics, it is different from the term 'weight' (though in other cases, they are interchangeable, like in chemistry).
W = mg
W = weight (in Newton)
m = mass (in kg)
g = acceleration due to gravity (on Earth) = 9.8 m/s^2
41 = m * 9.8
m = 4.18 kg
On Moon, we'll solve for acceleration due to gravity of moon:
81.5 = 4.18 * g,moon
g,moon = 19.5 m/s^2
How much lighter is he on the moon? If it means the mass, then his mass is constant and he is neither heavier nor lighter on the moon.
#2. Given his weight on the moon, solve for the mass using the g,moon.
90 = m * 19.5
m = 4.62 kg
Thus, his weight on the earth is
W = 4.62 * 9.8
W = 45.3 N
Hope this helps :3
Thank you so much Jai. You explained it very well:)