BUT! Mary is NOT! Here is here comment.
Okay, assuming 6" is the finished edge of your equilateral triangle, and a width of 108".For each row, you will need 35 triangles (18 with base at the bottom, 17 with base at the top) and 2 half equilateral triangles for each end. This works out to exactly 108" finished.For the height, it is a bit trickier. The Pythagorean theorem kicks in here. The height with a six inch base would be almost 5.2 inches. So twenty rows would be about 104 inches. Using those dimensions, you would need 700 triangles and 40 half equilateral triangles. Is that the way you were thinking? Hope this helps.
WhooooHOOOO! Now if I could get someone to CUT all those triangles!