Divide each cost over the number of bottles
For the 6-pack, we have
3.79 dollars divided by 6 = 3.79/6 = 0.6317 which rounds to 0.63
So we pay $0.63 per bottle or 63 cents per bottle
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For the 9-pack, we have
4.50 dollars divided by 9 = 4.50/9 = 0.50
meaning we pay $0.50 per bottle or 50 cents per bottle
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For the 12-pack, we have
6.89 dollars divided by 12 = 6.89/12 = 0.57416 which rounds to 0.57
So we pay $0.57 per bottle or 57 cents per bottle
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In summary we have
The 6-pack costing 63 cents per bottle
The 9-pack costing 50 cents per bottle
The 12-pack costing 57 cents per bottle
The lowest per bottle cost is 50 cents per bottle, which is for the 9-pack
The 9-pack is the best deal of the three deals.