Question

a grocery store sells 6-pack of bottled water for $3.79 a 9 pack for $4.50 and a 12 pack for $6.89 which package costs the least per bottle? explain your reasoning

Answer 1

Find the unit price (price per bottle) 
$3.79 ÷ 6 = $.63 a bottle (approximately)
$4.50 ÷ 9 = $.50
$6.89 ÷ 12 = $.57 (approximately)

So your best bet is the 9 pack.  
You will find that sometimes your best value is not to buy the greatest amount of a product.