Since the Amy’s age is twice the sum of Maggie and Karen’s ages, we conclude that the sum of M and K ages is one third of 105, i.e. 35 years, and the Amy's age is 35*2 = 70 years. Next, we have M + K = 35, (1) M = 3K - 25 (2) So, we substitute (2) into (1) by replacing M there, and we get one single equation then (3K - 25) + K = 35, 4K = 35 + 25 4K = 60 which has the solution K = 15.