Probability

[frog0101]

Hi,
I have been struggling with this question:
A fair coin is tossed n times. What is the probability of heads on the
first toss given that r heads were obtained in the n tosses?

The solution is r/n, but am not sure how to get there.

Any help would be great, thanks.

[RuiAce]

This does not look like HSC related material so I won't address it any further here. (You can post in the first year uni maths discussion thread, or label that this is an extracurricular question in the future if that is what it is.)

However having said that, allowing for beyond HSC maths concepts, I'd think Bayes rule is a good idea here. (Edit: Can confirm that Bayes rule works.)