1) $6 for 8 ounces
Value per ounce = 6/8 = 0.75
You are paying $0.75 per ounce of shampoo.
2) $4 for 5
Value per ounce = 4/5 = 0.80
You are paying $0.80 per ounce of shampoo.
Comparing both value per ounce: (0.75 < 0.80)
You are paying more when using second bottle.
Bottle one is cheaper.
0.80 - 0.75 = 0.05
You are paying $0.05 more for bottle two.