Questions about EXCEL formula

Hi, 

I dont know if WSO is the right forum for this, but since there is an off-topic post, I thought I give it a try.

Anyways I have the following problem:

• I have a time table on excel
• The Intervals are:
  • y-achses = every 30 minutes
  • x-achses = every day
•  Inside the time table I have different categories (lets call them a, b, c)
• On the top of the page I have
  • required hours of a, b, c and
  • actual hours of a, b, c

Now what I want to do is, I want to make a formula that looks through the whole table and for every cell that has "a" in it, I want it to add 30min - and the final value of the cell shut be the sum of all the "a's" * 30 min