How to determine the percentage of iron(II) and iron(III) in a solution?
You’re already very close. You have realised that:


You can titrate the $\ce{Fe^{+II}}$ content with a potassium dichromate standard solution; and
You can turn $\ce{Fe^{+III}}$ into $\ce{Fe^{+II}}$ by adding metallic zinc.


Note that in the first bullet point of mine, your second reaction, you are creating $\ce{Fe^{+III}}$, hence iron(III) that was present before you started won’t be modified. Similarly for the other reaction and iron(II). That should lead us to following methodology, hidden in a spoiler tag:


   1. Titrate the iron(II) content in an aliquot of your solution. That gives you the content of iron(II) and only iron(II).

 2. Take another aliquot and reduce all iron(III) to iron(II). That will give you the total iron content.

 3. Substract the value from 1. from the value from 2. to get the content of iron(III).