The simplest form of the ratio 81:45 is:

The simplest form of a ratio is a form that consists of a co-prime numerator and denominator. This cannot be further simplified.

We know that $\frac{81}{45}=\frac{9\times 9}{5\times 9}$

Therefore, $\frac{81}{45}=\frac{9\times \cancel{9}}{5\times \cancel{9}}$

                            $=\frac{9}{5}$

9:5 cannot be divided further. So it is the simplest form.