To find the average number of customers for dinner, use the simple ratio of 5 lunch customers for every 8 dinner customers.
Because there are 40 lunch customers, this is eight groups of five lunch customers. This means you will need 8 groups of 8 dinner customers to make it equivalent.
Dilation changes (x,y) values not the grid or coordinate plane. Basically, dilating a graph or a coordinate grid means the original coordinates you may have had will be changed with the dilation. For example, a triangle plotted had its original area of 26 dilated to an area of 58.