Let the original concentartions be A and B, then the rate:

r = k A^2/B

Let the new concentartions be A' and B', then as B' is 100% more than B,

B'=2B, so if the rates befor and after are equal:

A^2/B = A'^2/B' = A'^2/(2B),

so A' = sqrt(2) A ~= 1.4 A, which is approximatly a 40% increase.

RonL