jmc

When we change from a 9-inch wrench to a 15-inch wrench, we multiply the length of the wrench by $\frac{15}{9}=\frac{5}{3}$. Since the wrench length and the force required are inversely proportional, their product must be constant. So, when we multiply the wrench length by $\frac{5}{3}$, we must multiply the force required by $\frac{3}{5}$ in order to keep their product constant. So, the force required is $(375)\left(\frac{3}{5}\right)=\boxed{225}$ pounds of force.