John is n years old now. How old was he 10 years ago? What will be his age in 20 years time?
10 years ago means we subtract 10 from n:
n - 10
20 years time or 20 years from now means we add 20 to n:
n + 20
10 years ago means we subtract 10 from n:
n - 10
20 years time or 20 years from now means we add 20 to n:
n + 20