what's the ratio of atoms of these two isotopes?
You need more information.  They probably expect you to look in a book and see that the atomic weight (which is a weighted average over all isotopes) of $\ce{Tl}$ is 204.383.  Now if x is the amount of $\ce{^203Tl}$ with atomic weight 202.97 and (1-x) is....

Does that get you started?

$$\ce{(x*202_{.}97) + ((1-x)*204_{.}97)~=~?}$$

Can you get to the answer from here?