Income of X is 25% more than that of the income of Y. What percent is Y's income less than that of X's income?

Let the income of Y be 100.
Since the income of X is 25% more than that of Y:

$$\text{Income of X}=100+25=125$$

The amount by which Y's income is less than X's income is:

$$125-100=25$$

To find the percentage by which Y's income is less than X's income, we compute:

$$\text{Percentage Less}=\left(\frac{25}{125}\right)\times 100=20\%$$