Assume value for B

Let the value of $B = 100$ to easily compute percentages.

$B = 100$

Determine value of A

Since $A$ is 40% more than $B$, $A = 100 + 40 = 140$.

$A = 140$

Calculate the difference

The difference between $A$ and $B$ is $140 - 100 = 40$.

$A - B = 40$

Compute percentage less with respect to A

B is less than A by the difference divided by A: $\frac{40}{140} \times 100 = \frac{2}{7} \times 100 \approx 28.57\%$.

$\frac{40}{140} \times 100 = 28.57\%$