by switching you double the probability of finding the car voucher from 1/3 to 2/3. Switching is only not advantageous if you initially choose the 'right' box, which happens with probability 1/3.
With probability 1/3, you initially choose one of two 'wrong' boxes; when the other 'wrong' box is opened, switching yields the 'right' box with more certainty. The total probability of winning when switching is thus 2/3.
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