Count column blanks IF adjacent column is NOT blank AND another column IS also blank!

Hi! It's my last day at work and I have been DESPERATELY trying to finish off a project I've been working on here with an excel database, I've been tearing my hair out over this for days, if someone can give me the magic formula in the next 7 hours (wishful much?!) that would be amazing!!

I have something like this:

Column A Column B Column C
(Letter sent) (forms returned) (Review completed)

I need to know how many forms we are waiting to receive (column B blanks) if a letter has been sent out (column A NON blanks). But there is a spanner in the works!!! Sometimes the review has already been completed (column C) and even though a letter has been sent, we now do not care if the forms aren't returned since the review has been done. So in that instance I need something like; count blanks in column B only if column A HAS data AND if column C is blank

If someone has a magic formula before I leave work today I'll be so grateful (as will my employers and the many people who will be using this spreadsheet if I can get it to work as it should!).

I have tried so many COUNTA, COUNTIF (AND (IF etc formulas that my brain is being fried!!! None have worked!! Thanks to anyone who can save me!!!!

***NOTE as an aside, while tagging this post I noticed the "countcolour" tag...I'm about to go search on this because it would be an easy solution to my problem as all of the letter sent rows have been coloured yellow and if I can just locate and count all yellow rows it would solve this nightmare for me!

Oh my goodness! BOTH of those work perfectly!!! Thank you both so much! I can't believe how much time I spent scouring forums yesterday and playing with different formulas trying to get something to work and now you have both responded within minutes with a perfect solution!!! thank you so so much!!

Just wondering, between the two formulas, is there one that may work better than the other? As I will be leaving after today, this spreadsheet needs to work smoothly for the other people in the office as they are not very excel-literate and will not be able to fix things if they go awry...