function to sum a column's entries based on another column

Hello. Thanks for reading. I am grateful for your expertise.

I'm an intermediate excel user, and comfortable with nested if statements and other functions, but there's one task I repeatedly come across that I don't know how to do efficiently.

Let's say I have a dataset with blank or X in column A, and a dollar value in column B. I want to sum the dollar amounts for column B only if there is a corresponding X in that row in column A. The way I currently do that is by creating a new column C, making a if statement to display the value of B only if there is an entry in A, and then doing operations on that new range. I figure there has to be an easier way. I don't know how to use VLOOKUP, despite trying to read the helpme a bunch of times. Is that relevant?

Maybe like this in one cell:

=SUMIF($A$1:$A$100,"="&"X",$B$1:$B$100)
...or
=SUMIF(A1:A100,"="&"X",B1:B100)

You mean you just want a count of how many Xs there are?

=COUNTIF($A$1:$A$100,"="&"X")

Um, you want a count of how many times an X occurs in one column, but only if there is also an X in a previous column?

That's something like this, maybe:

=SUMPRODUCT(--($A$1:$A$100="X"),--($B$1:$B$100="X"))

If that takes care of your need, be sure to EDIT your original post (Go Advanced) and mark the PREFIX box [SOLVED]