Paul is 14 years old.
His sister is exactly 6 years younger, so this year she is 8 years old.
This year, the ratio of Paul’s age to his sister’s age is 14 : 8

14 : 8 written as simply as possible is 7 : 4

When Paul is 21, what will be the ratio of Paul’s age to his sister’s age?
Write the ratio as simply as possible.