What is the expected number of rolls till a 1 comes up on a standard 6-sided die, conditional on only odd numbers coming up?

First, without the conditional, the answer would be 6 rolls. (This is the expected value of a Geometric distribution, if you want to get very technical, which of course we do.)

What about with the conditional? One way you might reason is like so: Only getting odd rolls is like having a 3-sided die, with just the 3 odd numbers: 1, 3, 5. So for the same reason the answer is 6 with with 6-sided die, it's 3 with this hypothetical 3-sided die.

I would've fallen for that but now believe it's wrong. We can discuss further in the comments.

This market resolves to whatever a proof plus a simulation both agree on.

Source: Stan Wagon