With clear: none, the first and last paragraphs' margins collapse and the last paragraph's top border edge should be flush with the top of the floating paragraph.

With clear: left, the top border edge of the last paragraph must be below the float, i.e., 50px lower. That means that the margins must not collapse and clearance must be added such that clearance + margin-top = 50px, i.e., clearance = 50px - margin-top = 50px - 75px = - 25px.