Answer:
200 N 
Step-by-step explanation:
F = G m1 m2 / r^2 G m1 and m2 are constant for this scenario
 so this becomes 
F is proprtional to 1/r^2 
f = 1/r^2 
800 = 1/r^2 now change r to 2r 
 F = 1/(2r)^2 = 1/4 * 1/r^2 <======shows force will be 1/4 of original , or 200N