maybe I haven't read all the posts but my question is it cheaper for me to buy 80lb bags or mix it myself with 90 lb bags of portland sand-gravel mix, most lilkely 7/8 gravel.
I have a 6ft x 18 foot slab with steel mesh and reinforce rebar 12 1/2 inch.
then I have a 10ft x ten ft.slab for a garage front.
and finally a 4ft x 6 ft door slab?

Neither.

It will be cheaper if you simply call a Redi-Mix concrete truck.

Not only will Redi-Mix generally cost less, you'll get a stronger concrete mix and you'll save all the backbreaking labor that goes with picking up, delivering and mixing the 3 to 4 1/2 yards of concrete (depending on slab thickness) manually.