The Addition Rule

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Hook

The card that gets counted twice

What's the chance a drawn card is a king OR a heart? There are 4 kings and 13 hearts — but the king of hearts is in both lists. Add 4 + 13 and you've counted that one card twice. The addition rule is the fix: add the pieces, then subtract the overlap.

Whenever 'or' events can both happen, subtract the part they share.

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