Question

# What resistance must be placed in parallel with a 155 Resistor to make the equivalent resistance 115 ?

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What resistance must be placed in parallel with a 155 Resistor to make the equivalent resistance 115 ?

$$\displaystyle{\frac{{{1}}}{{{R}}}}={\frac{{{1}}}{{{r}_{{1}}}}}+{\frac{{{1}}}{{{r}_{{2}}}}}$$
$$\displaystyle{\frac{{{1}}}{{{115}}}}={\frac{{{1}}}{{{155}}}}+{\frac{{{1}}}{{{r}_{{2}}}}}$$
$$\displaystyle{\frac{{{1}}}{{{r}_{{2}}}}}={\frac{{{1}}}{{{115}}}}+{\frac{{{1}}}{{{155}}}}$$
$$\displaystyle={\frac{{{40}}}{{{115}\times{155}}}}$$
$$\displaystyle{r}_{{2}}={445.625}$$ Ohms