I need a vector/trigonometric solution to the following problem: 
Given a stationary target and a mobile firing platform with the
ability to "lock" on to the target, how do you calculate the
adjustment in firing angle (lag) needed to compensate for the velocity
of the mobile platform.

Constriants:  the mobile firing platform's movement is contained
within a 54' by 26' rectangular field with the target fixed in the
center of one 26' side. The target is 30" in diameter and centered 10'
above the ground.  The projectile is a 7" diamter nerf type foam ball
fired at a maximum muzzle velocity of 30 ft/sec.  Maximum practicle
firing range is 35'.  The maximum speed of the mobile firing platform
is unkown, but will not exceed 15 feet/sec.

Please address the top-down 2-D problem first.

This question stems from a high school robotics project.  The solution
is needed for educational purposes.

Nice, but I need more data.
First of all it does not matter if the target is moving and platform
is not movable or the other way around. The data that are important
are distance to the target 35'. the speed at which the target or the
platform moves (15 feet/sec). and the average speed of the projectile
hits the target. I can calculate that from the maximum velocity (30
ft/sec) but then I need the air resistance from the ball. Can you
supplie me with these data? Or do you want me to get these data?
PLease mail me
Prof. kees