Can you help me with a math problem?
 

I have a 78" quilt edge. I want to add a half square triangle border of about six inches wide. How do I figure the size of the squares to cut HST's to fill a length equal to 78"? I sure hope I explained that the right way. Thanks

I'm sure someone will respond to you soon, but I just wanted to let you know that your title made me laugh. It's amazing to me that doing something as creative as quilting can involve so much math (which I hate!). Good luck with your question.

If you are making 6" HST (I assume finished size which means 6.5" unfinished)
then you will need to cut your squares 7.5", sew then trim to 6.5" square.
By cutting 7.5" you get a little wiggle room. At least that's the way I do it.

13 6" (finished) HST's will cover exactly 78 inches. So you can make 52 for all 4 sides, and if you want them in the corners too they will fit exactly - so another 4 HST's for a total of 56 HST's. Lucky you that everything will fit so nicely! That's not the way it usually goes.