How thick does steel need to be to stop these bullets?

If Professor Fondsworth were an actual ballistics expert he would have asked some basic questions. Like what is the distance to the target, what type of round is being fired (FMJ, AP, etc.), what is the mass of each bullet, what grade of steel plate, what angle is the steel plate in relation to the impact, etc. These are all factors that MUST be considered when trying to stop a bullet from penetrating a steel plate.