I have a big excel sheet with different colums headings.
at the bottom i wanted a conditional sum basing on 3 columns
May be a example of my question be like as follows

column a column b column c
(town) (description) (volume)

vijayawada sales 23
guntur credit sales 27
vijayawada sales 30
guntur credit sales 40
nellore sales 55
vijayawada credit sales 25

computer has to sumup the corresponding values of sales and credit sales
separately for vijayawada, guntur, nellore

thanks in advance