For the answer to the question above, \
Let
D = Dominic weight
R = Ralph weight
P = Peter weight
We have three unknowns so we should set up three equations. We do as follows:
D = 0.5R
P = 3D
D+R+P = 720
Solving simultaneously we obtain,
D = 120
R = 240
P = 360
So the answer is 120 is the weight of Dominic
