To find a unit price, you need to divide. ($3.79 by 6, $4.50 by 9, etc.)
Once you find the answers to the division problems, pick the one which is the least, which is $0.5 out of the others which were $0.63 and $0.57. Therefore, the 9 pack for $4.50 package is the least per bottle, $0.5 per bottle to be exact.
