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.