Question: In 10 years, Melanie will be 18 years old. In how many years will her age be thrice her present age?
Answer: 
Melanie's present age is 18 - 10 = 8.
Thrice Melanie's present age is 8 x 3 = 24.
So, she will be thrice her present age in 24 - 8 = 16 years.