Kathryn's age was 3 times Cody's age. How old is Kathryn now? The student writes out his solution, noting what he did: Cody now = c, so Cody 7 years ago was c - 7 Kathryn now = k, so Kathryn 7 years ago was k - 7 Cody is 44 years younger than Kathryn, so c = 44 - k 7 years ago, Kathryn's age was 3 times Cody's age k-7 = 3(c - 7) k-7 = 3[44 - k - 7] Substitute for c using c = 44 - k k-7 = 3[-k + 37] Simplify k-7 = -3k + 111 4k = 118 k = 29.5 There is no way that Kathryn's age is 29.5 if Cody is supposed to be 44 years younger than Kathryn, so what went wrong? What mistake did the student make?