First off, calculate for the remaining cases that need to be produced (x). This is shown below.
x = 68500 - 11775 = 56725
Since the rate production is only 4200 cases per hour, divide the numbers of cases that needs to be produced by this rate to get the time (t).
t = x / 4200 = 56725 / 4200 = 13.506 hours
Thus, it will take about 13.506 hours to produce the remaining cases. This is approximately 13 hours and 30 minutes.