Partial Derivatives & the Gradient · Challenge problem

Two resistors with resistances $R_1 = 3$ ohms and $R_2 = 6$ ohms are connected in parallel, giving total resistance $R = \dfrac{R_1R_2}{R_1 + R_2}$. $R_1$ is measured with a relative error of at most $2\%$ and $R_2$ with a relative error of at most $3\%$. Use differentials to find the largest possible relative error in $R$, as a percentage. Give the number of percent as an exact fraction (without the $\%$ sign).