Successive discounts of $x\%$ and $y\%$ are equivalent to a single discount of:

Let the initial marked price be $\text{Rs. }100$.
* After the first discount of $x\%$, the price becomes:

$$100\times \left(1-\frac{x}{100}\right)=100-x$$

* After the second successive discount of $y\%$, the final price becomes:

$$(100-x)\times \left(1-\frac{y}{100}\right)=100-x-y+\frac{xy}{100}=100-\left(x+y-\frac{xy}{100}\right)$$

The total reduction from the initial value of $100$ is:

$$100-\left[100-\left(x+y-\frac{xy}{100}\right)\right]=x+y-\frac{xy}{100}$$

Thus, the equivalent net single discount percentage is $\left(x+y-\frac{xy}{100}\right)\%$.