Formula/Algebra help?

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?

Sorry. I thought I may have posted it in the wrong forum (as there were no responses, and I assumed this would be an easy one for you excel gurus).

As for a homework assignment... I wrote it out in the best way I could to be clear on what kind of formula or function I was looking for that could help me learn how excel could best accomplish my goal. I'm building a material list (for our salesmen) that auto calculates the cost of products per vendor and auto-selects the most cost effective vendor based on the quantities necessary input by the salesman.

I'm guessing Kaper's deleted post had a correct solution, but Kaper and FDibbins both appear to be offline, and you've clarified that this isn't homework, so here's what I have:

If your 5 lb. cost is in column A and your 30 lb. cost is in column B, then place the formula below in C2 and fill down through all of your cost combinations. If the 30 lb option is blank, it'll give you the cost of seven 5 lb. boxes. Otherwise, it'll return the cost of a 5 lb box and a 30 lb box.

=IF(ISBLANK($B2),7*$A2,$A2+$B2)

If you're planning heavy modifications and want something more flexible, the formula below ultimately does the same calculation, but you can clearly see the 5,30, and 35 so that you can edit those if your numbers change. You could also tie those values to cells if you want to change the numbers on the fly. Enter it in C2 and fill down.

=IF(ISBLANK($B2),$A2*35/5,INT(35/30)*$B2+(MOD(35,30)/5)*$A2)