In four years, Peter will be twice as old as Harriet. If Peter's age is currently half of his mother's age, who is 60, how old is Harriet now?
Thoughts? Step-by-step reasoning:
If Peter's age is currently half of his mother's age, who is 60, Peter is 1 / 2 * 60 = 30 years old. In four years, Peter will be 30 + 4 = 34 years old. Since Peter's age in four years will be twice Harriet's age, Harriet will be 34 / 2 = 17 years old in four years. Currently, Harriet is 17 - 4 = 13 years old.
Thus, the answer is 13.