Answer:
We might expect a foot traffic at the 9 minute mark.
Step-by-step explanation:
We may expect approximately that each new customer arrives at a rate of 60/6 = 10 seconds. The two vendors may serve a total of 4 customers per minute, so we may assume that each minute 2 more customers will be kept waiting. We need more than of 15+2 = 17 customers to be waiting (this includes the 2 ones being served). Thus, we might expect a foot traffic at the 18/2 = 9 minute mark.