Sum Values in One Range Based on Criteria in Another

I have a simple spreadsheet, two columns "Amount" (col Q) and "Account" (col O). I want to get the sum of the "Amount" column where the corresponding "Account" forum equals "Paypal".

This is as far as I can get!

=SUM(Q:Q)*(O:O="Paypal")

I believe rather than using SUM() you want the SUMIF() function. I don't have Excel to hand but I believe you need your formula to be

=SUMIF(O:O,"Paypal",Q:Q)

From this site

The syntax for the SumIf function is:

SumIf( range, criteria, sumRange )

range is the range of cells that you want to apply the criteria against. (your column containing "Paypal"

criteria is used to determine which cells to add. - In your case matching "paypal"

sumRange are the cells to sum.

Also see this Microsoft help page, and this page which has some more advanced functions such as DSUM and SUMPRODUCT.