What's My Rule?

Each pair of numbers are related by some kind of rule.
All of the pairs  follow the same rule.

a. Tell me what the rule is.
b. What do you think the 2nd number is in the unfinished pairs?

3 is paired with 8                  7 is paired with 14
1 is paired with 4                  5 is paired with 12
6 is paired with 14                0 is  paired with 2

40 is paired with ??               9 is paired with ??