It's February 2021. Mark was born in January 1976. Graham is 3 years younger than Mark, and Graham's sister, Janice, is 1/2 the age of Graham. How old is Janice? Let's be accurate as possible.
It's 2021 and Mark was born in 1976 so Mark is 2021 - 1976 = 45 years old. Graham is 3 years younger than Mark who is 45 so Graham is 45 - 3 = 42 years old. Janice is 1 / 2 the age of Graham who is 42 so Janice is 42 / 2 = 21 years old.
The answer: 21.