The data in figure 31.13 shows that from hour-to-hour, the pollen tube elongates about 80 microns per hour (80 microns at the end of one hour, 640 microns at the end of 8 hours): for any hour, divide the length by that number of hours, and you always get 80 microns/hour. This matches the number in the table.
Work Step by Step
Rates are change per time, so divide the change between any two hours by the amount of hours between them. The answer in this case is always 80 microns/hour.