For Enrique a 12 ounce bottle lasts for 16 weeks. This is a known fact. We need to find the amount of days or weeks an 18 ounce bottle of the same brand of shampoo last. Now 12 ounce bottle lasts = 16 weeks = (16 * 7) days = 112 days 18 ounce bottle lasts = (112/12) * 18 = 168 days = 168/7 weeks = 24 weeks So 18 ounce of the same brand of shampoo will last for 24 weeks.