Let's solve this down step by step:

1. A is 40% less than B: This means A = 0.6B (since 100% - 40% = 60%)

2. C is 40% of the sum of A and B: This means C = 0.4(A + B)

Now, substitute the value of A from step 1 into the equation for C:

C = 0.4(0.6B + B)

C = 0.4(1.6B)

C = 0.64B

To find the percentage by which B is greater than C:

(B - C) / C × 100%

= (B - 0.64B) / 0.64B × 100%

= 0.36B / 0.64B × 100%

= 56.25%

So, B is greater than C by 56.25%.