Help in 'Challenge: Wait times' in unit 3, lesson 3

I don't get 'Challenge: Wait times' in unit 3, lesson 3.

So far I have:

import random

num_cashiers = 1

waiting_to_order = 0
waiting_for_food = 0

for waiting_to_order in range(300):
    # Customers arrive every minute and line up to order.
    waiting_to_order = waiting_to_order + random.randint(0, 6)

    # Each cashier can take up to three orders per minute.
    new_orders = min(3, waiting_to_order)

    # After ordering, customers wait for their food to be made.
    waiting_to_order = waiting_to_order - new_orders
    waiting_for_food = waiting_for_food + new_orders

    print(str(waiting_to_order) + " customers waiting to order.")
    print(str(waiting_for_food) + " customers waiting for food.")

Can someone please help me? Thanks in advance.