Which function will shorten IF-THEN-ELSE ?
 

Here's a simplified example of what I want to do:

Col A Col B
Row 1 yes 5
Row 2 10
Row 3 yes 15
Row 4 yes 20

What is the formula that will automatically find all numbers in Col B
that have a corresponding "yes" in Col A, then SUM them and multiply
by 30% ? If done manually, the final result would then be the
equivalent of =(5+15+20)*.30, which is 12.

Thanks,