This block, 3 cm long, 2 cm deep and 4 cm tall, is painted yellow on all six faces and cut into 1 cm cubes. How many of the cubes have exactly one painted face?

Every cube touches the front or the back, because the block is only 2 cubes deep. A cube has just that one painted face if it is also in the middle column (1 of 3) and not in the top or bottom layer (2 of 4): $1\times 2\times 2=4$. (12 is the number of two-face cubes and 8 the number of corners.)