## Elementary Technical Mathematics

$$142.857 \ \Omega.$$
We have $$\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}=\frac{1}{225}+\frac{1}{475}+\frac{1}{ 925}\\ =0.004+0.002+0.001=0.007$$ Hence, we get $$R=\frac{1}{0.007}=142.857 \ \Omega.$$