Which is a better deal? AND why?... a 2 liter bottle of Coke for $1.99 or a 20 oz bottle of Coke for 89 cents.

1 liter = 33.814 ounces, so 2 liters = 67.628 ounces

Number of ounces per dollar:  

For the 2 liter bottle:  67.628 ounces / $1.99  =  33.98 ounces per dollar  

For the 20 ounce bottle:  20 ounces / $0.89  -  22.47 ounces per dollar

You get more ounces per dollar with the 2 liter bottle.