If by "1/2 falling" you actually mean "1/2" gallon of soda left, we can use that and make the denominator of 1/2 to 16.
1/2 * 8/8 = 8/16
Since Steve has 8/16 gallons of soda left and each soda has 1/16 gallon, Steve can sell 8 more sodas.
Hope that helped!