Question

if a person weight is 750 N on the earth how much would be his weight on the moon given that moons mass is 1÷81of that earth and its radius is 1÷3.7 of that of earth?

Answer 1

I think you should apply the formula  g=GM/$R^{2}$
on surface of moon , then compare g/g' like  
g/g'=GM/$R^{2}$  / GM'/$R'^{2}$ 

and you can also multiply g/g' with m on both sides to directly put the value 750N.

Though what i have written might look difficult to understand but give it a moment and understand what i am trying to say.

Hope it helps!