John's age is currently $3$ times Mary's age. In $10$ years, John's age will be $2$ times Mary's age. How old is **Mary now**?

Let Mary's age $= m$. John's age $= 3m$. In $10$ years: John $= 3m + 10$, Mary $= m + 10$. New ratio: $3m + 10 = 2(m + 10)$. $3m + 10 = 2m + 20 \Rightarrow m = 10$. Check: now John $= 30$, Mary $= 10$ ($30 = 3 \cdot 10$ ✓). In 10 yr: John $= 40$, Mary $= 20$ ($40 = 2 \cdot 20$ ✓). - Trap D ($30$) is John's current age. - Trap C ($15$) is Mary's age in 5 years.