Let extra share should he buy be $\mathit{x}$ .
then total number of shares $=50+\mathit{x}$
Total face value $=\mathit{₹}100\times \left(50+\mathit{x}\right)$
∴ Annual income $=\mathit{₹}\frac{100\times \left(50+\mathit{x}\right)\times 7.5}{100}$
$=\left(50+\mathit{x}\right)\times 7.5$
$\therefore \left(50+\mathit{x}\right)\times 7.5=375+150$
$50+\mathit{x}=\frac{525}{7.5}=70$
$\mathit{x}=70-50=20$
Hence, the extra shares should be buy $=20$.