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Let x be Bob's current age. Then Karl's current age is 2x. Nine years ago Bob's age was x-9, and Karl's age was 2x-9. We're told that nine years ago Karl's age (2x-9) was three times Bob's age (x-9). Therefore, 2x-9 = 3(x-9) = 3x-27. Subtract 2x from both sides, and add 27 to both sides: 18 = x. So Bob's current age is 18, and Karl's current age is 36, twice Bob's current age. (Nine years ago Bob would have been 9, and Karl would have been 27, or three times Bob's age then.)

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