Unless this is supposed to be just an exercise practising basic arithmetic, I'm not convinced that considering semi-bluffing whilst ignoring implied odds has that much value. I know you want to keep things simple, but I can't help thinking if people haven't got a grasp of how to do these kind of simple calculations either by themselves or after quite a number of carefully explained examples by yourself then they're not going to get it this time. It seems a shame to talk about semi-bluffing without discussing how it affects your opponent's range, your implied odds and basically all the interesting stuff.