worksheet function - How to assign rows in multiples of a number (Excel/Google Docs)

For example, I want the first entry in Column B (B1) to show A5, then B2 shows A10, then B3 shows A15.

How would I go about doing this?

Answer

For excel you can put this in B1 and drag down

=INDEX($A$1:$A$100,5*(ROWS($B$1:B1)-1))

To start with A5 instead of A1, use this

=INDEX($A$1:$A$100,5*(ROWS($B$1:B1)))