Logic: 25% of blue furbils are conical, and half of all conical furbils are blue. One quarter of all furbils are neither blue nor conical. What percentage are BOTH blue and conical?

From NCTM

Solution: the non-conical blue furbils outnumber conical ones 3 to 1, but the non-blue conical furbils are as common as the blue ones. For example, if there are 3 blue conical furbils, there must be 9 non-conical blue ones, and 3 non-blue conical ones. Since 3/4 of all furbils are either blue or conical, there must be 5, in this example, which are neither. Of the 20 altogether, 3 are both, which gives 15%

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