Let x be the sister age. Then the mom's age is 3x, according to the condition. In 12 years the sister will be x+12 years old, while the mom will be 3x+12, and the condition says that then 2*(x+12) = 3x + 12. Simplify and solve: 2x + 24 = 3x + 12 24 - 12 = 3x - 2x ====> x = 12. Answer. The sister is 12 years old; the mother's age is 3*12 = 36 years.