Answer:
Mary is 36 years old
Her son is 12 years old
Explanation:
Let's assume, that Mary's son is x years old and Mary is 3x years old
Let's write 2 equations according to the given information:
x = 3x (Mary is 3 times older than her son)
3x + 12 = 2(x + 12) (after 12 years, she will be 2 times older than her son)
3x + 12 = 2x + 24
3x - 2x = 24 - 12
x = 12
That means, Mary's son is 12 years old
Mary is 3 × 12 = 36 years old