To be able to calculate the problem above, we need to set up equations. We 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
Therefore, Dominic weighs 120 mass units.
