A rational number is any number that can be written as the ratio between two other numbers i.e. in the form 
Part A:
An easy choice that makes sense is 7.8, right in the middle. To prove that it's rational we need to write it as a ratio. In this case we have
Part B:
We need a number that can't be written as a ratio (because it neither terminates nor repeats). Some common ones are
, 
, 
and 
so it makes sense to try and use those to build our number. In this case 
works nicely.
Part A:
An easy choice that makes sense is 7.8, right in the middle. To prove that it's rational we need to write it as a ratio. In this case we have
Part B:
We need a number that can't be written as a ratio (because it neither terminates nor repeats). Some common ones are