Re: Typo in Sklansky Column
There are 30 children in your child's class. How many are either boys or redheads? If I tell you that there are 13 girls and four redheads, that doesn't mean the answer is 17.
No, why would anyone think it is 17? One might naively think it is 21, since this is 17 boys + 4 readheads. But then you would have to subtract the redheaded boys from 21 not 17. Clearly this is a typo and both bold words above should be boys. Then you subtract redheaded boys from 17 to get the final answer. This is how the demonstration of the inclusion-exclusion principle should be done.
boys OR redheads = boys + readheads - redheaded boys
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