When two resistors of $(600\pm 3)$ohm and $(300\pm 6)$ohm are connected in parallel then the equivalent resistance will be 

$\begin{array}{l}{\mathrm{R}}_1=600\pm 3\mathrm{\Omega };{\mathrm{R}}_2=300\pm 6\mathrm{\Omega }\\ {\mathrm{R}}_{\mathrm{P}}=\frac{{\mathrm{R}}_1{\mathrm{R}}_2}{{\mathrm{R}}_1+{\mathrm{R}}_2}=200\mathrm{\Omega }\\ \frac{1}{{\mathrm{R}}_{\mathrm{P}}}=\frac{1}{{\mathrm{R}}_1}+\frac{1}{{\mathrm{R}}_2}\Rightarrow \frac{{\mathrm{\Delta R}}_{\mathrm{P}}}{{\mathrm{R}}_{\mathrm{P}}^2}=\frac{{\mathrm{\Delta R}}_1}{{\mathrm{R}}_1^2}+\frac{{\mathrm{\Delta R}}_2}{{\mathrm{R}}_2^2}\end{array}$
$\begin{array}{l}{\mathrm{\Delta R}}_{\mathrm{P}}=200\times 200\left[\frac{3}{600\times 600}+\frac{6}{300\times 300}\right]\\ =4\left[\frac{1}{12}+\frac{2}{3}\right]=3\\ \frac{{\mathrm{\Delta R}}_{\mathrm{P}}}{{\mathrm{R}}_{\mathrm{P}}}\mathrm{\%}=\frac{3}{200}\times 100=1.5\mathrm{\%}\end{array}$
$\therefore$The equivalent resistance =200 ± 1.5 %