Dominic ralph and peter have just eaten a huge christmas meal dominic now weighs half as much as ralph and peter weighs three times as much is dominic together they weigh 720 pounds how much does dominic go away

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