Customers arrive at a bank drive-up window every 6 minutes based on a Poisson distribution. Once they arrive at the teller, service time is exponentially distributed based on a rate of 5 minutes per transaction

1. What is the probability that 3 or fewer customers will arrive in one hour?

Can I please get this answer explained to me without using lambda . I would like this answer using number only. I dont understand the way the answers are written in the answers on this website. please use numbers only