2d game : fire at a moving target by predicting intersection of projectile and unit

Okay, this all takes place in a nice and simple 2D world... :)

Suppose I have a static object A at position Apos, and a linearly moving object B at Bpos with bVelocity, and an ammo round with velocity Avelocity...

How would I find out the angle that A has to shoot, to hit B, taking into account B's linear velocity and the speed of A's ammo ?

Right now the aim's at the current position of the object, which means that by the time my projectile gets there the unit has moved on to safer positions :)