I need an equation for a website I'm programming

Inputs = 4, 6, 8, 10, 12, 14, 16

b = 3, 4, 4, 5, 5, 5, 5

c = 7, 12, 15, 20, 24, 28, 31*

When 'a' is inputs then I need 'b' and 'c' to equal the number in the same order as the 'a' inputs.

Ex. If a = 4, then b = 3 and c = 7 If a = 6, then b = 4 and c = 12

*not a typo