If 91% of A is 39% of B, and B is x% of A, then the value of x is:

$$\eqalign{ & 91\% {\text{ of }}A = 39\% {\text{ of }}B \cr & \frac{{91}}{{100}} \times A = \frac{{39}}{{100}} \times B \cr & B = \frac{{91}}{{39}}A \cr & B = x\% {\text{ of }}A \cr & \frac{{91}}{{39}} \times A = \frac{x}{{100}} \times A \cr & x = \frac{{91}}{{39}} \times 100 = \frac{{700}}{3} \cr} $$