what is the value of dplease provide the calculation process.)

The answer is d is equal to 0.15. I'm trying to figure out how to calculate it.

The manager of Petro North gasoline service station wants to forecast the demand for unleaded gasoline next month so that the proper number of gallons can be ordered from the distributor. The manager has accumulated the sales data and forecast accuracy measures during the past 10 months, which are shown in the table below