Question 1027232
First - the "back of envelope method" - this means we use estimates to help us understand the answer. 

If ice cream were 15 cents an ounce, 20oz would be $3.00, and 24oz, $3.60.

The 20oz container is less than $3.00 (therefore, less than 15c/oz)
and 
The 24oz container is more than $3.60 (therefore, more than 15c/oz)

so the 20oz is lower per ounce, and the better deal. 

Now, let's pull out the calculator 275/20= 13.75 cents an ounce. 425/24 = 17.71 cents an ounce, confirming the 20 ounce container is better.