Given,
MRP \[=\text{ }Rs\text{ }50,000\]
Discount % \[=\text{ }20percent\]
GST $=28percent$
Presently,
\[Discount\text{ }=\text{ }20percent\text{ }of\text{ }50,000\text{ }\]
\[=\text{ }\left( 20/100 \right)\text{ }x\text{ }50,000~=\text{ }Rs\text{ }10,000\]
In this way,
\[Selling\text{ }price\text{ }\left( discounted\text{ }value \right)\text{ }\]
\[\text{ }=\text{ }50,000\text{ }-\text{ }10,000\text{ }=\text{ }Rs\text{ }40,000\]
\[CGST\text{ }=\text{ }0\]
\[SGST\text{ }=\text{ }0\]
\[IGST\text{ }=\text{ }28percent\text{ }of\text{ }40,000\]
\[=\text{ }\left( 28/100 \right)\text{ }x\text{ }40,000\text{ }=\text{ }Rs\text{ }11,200\]
Hence, the amount of bill = Selling price + IGST
\[\begin{array}{*{35}{l}}
=\text{ }40,000\text{ }+\text{ }11,200 \\
=\text{ }Rs\text{ }51,200 \\
\end{array}\]