If $A=400$ and $B=600$, then $A$ is how much percentage less than $B$?

The difference between $B$ and $A$ is:
$$\text{Difference}=B-A=600-400=200$$
To find how much percentage $A$ is less than $B$, we compare the difference to $B$:
$$\text{Percentage Less}=\frac{B-A}{B}\times 100\%=\frac{200}{600}\times 100\%=\frac{1}{3}\times 100\%\approx 33.33\%$$