why is it 4?

The figure above shows the time it takes for each step at Randy's other Icecream parlor in a different town. A customer walks in and goes to the sampling section, places their order at step 2, step 3 is when the worker makes their icecream-step 3a is the amount of time it takes to make a milkshake, while step 3b is the amount of time it takes to make a sundae and step 4 is the amount of time it takes to check out. How many customers can randy serve in an hour at his ice cream shop? 4 customers per hour 3 customers per hour 8 customers per hour 6 customers per hour 5 customers per hour