A pupil writing about the probability scale says that 0.4, 1.2 and 0 can all be probabilities. Which one of the three numbers cannot be a probability?

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The scale stops at 1, the value of an event that always happens, so 1.2 would describe something more certain than certain and no experiment can produce it. Rejecting 0 instead is the common alternative, but 0 is the proper value for an event that never happens and belongs on the scale.