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Call Jim's weight $j$ and Bob's weight $b$. We can use the following system of equations to represent the information given: $$\begin{array}{rl}j+b& =180\\ b-j& =\frac{b}{2}\end{array}$$ Adding the two equations together gives $2b=180+\frac{b}{2}$. Solving for $b$ gives $3b=360$, or $b=120$. Thus, Bob weighs $\boxed{120}$ pounds.