Solution

\[ \begin{aligned} \text{M.P before GST} &= \text{₹}\,3500 \\ \text{GST} &= 12\% \\ \\ \color{green}\text{MP including GST } &= \color{green}\frac{\text{M.P}\times(100 + GST\%)}{100} \\[6pt] &= \frac{3500\times(100+12)}{100} \\[6pt] &= \frac{\cancel{3500}^{35}\times 112}{\cancel{100}_1} \\[6pt] &= 35\times 112 \\[6pt] \color{green} MP &= \color{green} \text{₹}\,3920 \\ D \% & = 20\% \\ \\ \color{green}\text{S.P} &= \color{green}\frac{\text{M.P}\times(100-D\%)}{100} \\[6pt] &= \frac{3920 \times(100-20)}{100} \\[6pt] &= \frac{3920 \times 80}{100} \\[6pt] &= 392 \times 8 \\[6pt] \color{green} SP &= \color{green} \text{₹}\,3136 \\ P\% & = 10\% \\[6pt]\color{green}\text{C.P} &= \color{green}\frac{\text{S.P}\times 100}{100+P\%} \\[6pt] &= \frac{3136\times 100}{110} \\[6pt] & = \frac{3136\times \cancel{100}^{10}}{\cancel{110}_{11}}\\[6pt] & = \frac{31360}{11}\\[6pt] \color{green} CP & = \color{green} \text{₹}\,2850.90 \end{aligned} \]

Answer Original CP \(=\ \color{red} \text{₹}\,2850.90 \) and SP including GST \(=\ \color{red}{\text{₹}\,3136}\)