We know that: d = r / 2, so: r = 2 d. Also: r = 2 d and d + r + p = 720 pounds, where: d stays for Dominic, r stays for Ralf and p for Peter. When we charge: r = 2 d and p = 3 d in the 3rd equation we have: d + 2 d + 3 d = 720. 6 d = 720; d = 720 : 6; d = 120. Answer: Dominic weighs 120 pounds. Hope this helps. Let me know if you need additional help!