Filling a colun with a series of numbers

Hello,

I am trying to copy a series of numbers that are labeled (example: 1, 1a, 1b, 1c) in 1 cell down a column as well as filling in the series of numbers (example: cell A1 - 1, 1a, 1b, 1c and cell - 2, 2a, 2b, 2c, etc) Can this be done? If so how?

Thanks so much
Marilyn