Dear student,
acceleration due to gravity is given by,
g= (GM/R)
M= mass of body
R= radius of the body
For asteroid,
g'= 0.5 x(GM/R)
as mass = mass of Earth/4
radius= radius of Earth/2
g'=0.5g
So weight of block on asteroid will be,
Weight= 40 x 0.5 x 10
Weight = 80N
Regards