# what's the ratio of atoms of these two isotopes?

here is a question says :

I know that atom's number is found by = $6.022*10^{23} * mol$

but how this formula is used here is what i dont know

what's the $idea$ of this question?

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