Speed of a Bullet

[tigerseye]

I've been stuck on this problem forever and I don't know how to do it if you can't use r as the radius.
A bullet is shot through two cardboard disks attached a distance D apart to a shaft turning with a rotational period T (see the attached picture). Derive a formula for the bullet speed v in terms of D, T, and a measured angle theta between the position of the hole in the first disk and that of the hole in the second.

[Tide]

You know the time required to make a full revolution and from the angle you can determine the time it takes the bullet to traverse the gap. Find the speed of the bullet by just dividing the length of the gap by the time it takes to cross it.

[Holdena]

I no this sounds stupid but how do you find how long it takes to cross the gap !

[zgozvrm]

If the bullet penetrated the second disk 45[itex]^\circ[/tex] later than where it penetrated the first disk, then the disks traveled 1/4 turn in the time it took the bullet to penetrate both disks, so...