1st Vendor sells 5LB Boxes at X
2nd Vendor sells 5LB Boxes at Y AND 30LB Boxes at Z (one 30LB box cost less than six 5LB boxes)

What formula would allow me to calculate for (example) a 35LB order?

1st Vendor obviously 5LB multplied by 7
2nd Vendor i need to auto calculate a purchase of one 30LB box and one 5LB box to get the most competitive cost.

Formula? Help?