(a)
If by "age in years" you mean that you only consider whole years, then this is not a one-to-one function, because you will have the same output for 365 inputs:
So, you can see that it is not true that every different input yields a different output.
(b)
This is a one-to-one function: since one mile is 1.609 kilometers, you can convert miles per hour into kilometers per hour by multiplying by 1.609:
x mph = 1.609x km/h
The inverse function would be
x km/h = x/1.609 mph.