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Answer:
Lisa is 5
Explanation:
When Lisa is 4 times her current age, 3 times her current age will have been added to her current age. That is, 15 years is 3 times her current age, so Lisa is 5.
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If you want an equation:
a + 15 = 4a
15 = 3a . . . . . . . . . subtract a
15/3 = a = 5 . . . . . Lisa's age is 5