If A's salary is $25\%$ more than that of B, then how much percent is B's salary less than that of A?

Let B's salary be $\text{Rs. }100$.Since A's salary is $25\%$ more than B's:
$$\text{A’s salary}=100+25=\text{Rs. }125$$
Now, we calculate how much B's salary is less than A's salary in terms of percentage:
$$\text{Difference in salary}=125-100=25$$
$$\text{Percentage difference relative to A}=\left(\frac{\text{Difference}}{\text{A’s salary}}\right)\times 100$$
$$\text{Percentage difference}=\frac{25}{125}\times 100=\frac{1}{5}\times 100=20\%$$