If the 20 fl.oz bottle contains 125% of the next smallest size, the next smallest bottle should be 4/5 of the 20fl. oz bottle.
That means the smaller bottle has 16 fl. oz (you can check it, multiply 16 by 1.25 and you get 20).
Next, you can find the price as dollars per fluid ounce.
The bigger bottle will be: 5.8/20=$0.29
The smaller bottle: 5.12/16=$0.32
In this case, the larger bottle is the better buy because you pay less per fluid ounce of lotion.
