Dominic+Ralph+Peter=720
so substituting in what we know (X stands for Ralph)
1/2X+X+3(1/2X)=720
simplify to
1/2X+1X+7/2X=720
get every coefficient on X with a denominator if 2 (so they can be added together wih least common denominators)
1/2X+2/2X+7/2X=720
add the left side of the equation up
10/2X=720
simplify
5X=720
divide five from both sides
X=144
recall that Ralph=X=144 pounds
Dominic=1/2(Ralph)=1/2(X)=1/2(144)=72 pounds
Peter=3(Dominic)=3(72)=216 poubds
Ralph=144 pounds
Dominic=72 pounds
Peter=216 pounds
