At $10\%$ discount the selling price of a toaster is $\text{Rs }18000$, what is the selling price if the discount is $37.5\%$?

Let the Marked Price ($\text{MP}$) of the toaster be $x$.At a $10\%$ discount, the selling price is $90\%$ of the Marked Price:
$$0.90\times x=18000$$
$$x=\frac{18000}{0.90}=\text{Rs }20000$$
Now, if the discount is $37.5\%$, the new selling price (${\text{SP}}_{\text{new}}$) will be:
$${\text{SP}}_{\text{new}}=\text{MP}\times (100\%-37.5\%)$$
$${\text{SP}}_{\text{new}}=20000\times 62.5\%$$
$${\text{SP}}_{\text{new}}=20000\times \frac{62.5}{100}=200\times 62.5=\text{Rs }12500$$