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....

Does that get you started?

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

Can you get to the answer from here?