A wholesaler sells a watch to a retailer at a gain of $20\%$ and the retailer sells it to a customer at a loss of $10\%$. If the customer pays $\text{Rs }874.8$, what had it cost the wholesaler?

Let the cost price for the wholesaler be $\text{Rs }x$.The wholesaler sells it to the retailer at a gain of $20\%$:
$$\text{Cost price for retailer}=x\times \left(1+\frac{20}{100}\right)=x\times \frac{120}{100}$$
The retailer sells it to the customer at a loss of $10\%$:
$$\text{Price paid by customer}=x\times \frac{120}{100}\times \left(1-\frac{10}{100}\right)=x\times \frac{120}{100}\times \frac{90}{100}$$
Given that the customer pays $\text{Rs }874.8$:
$$x\times \frac{6}{5}\times \frac{9}{10}=874.8$$
$$x\times \frac{54}{50}=874.8$$
$$x=\frac{874.8\times 50}{54}$$
$$x=16.2\times 50=810$$
Thus, the cost to the wholesaler was $\text{Rs }810$.