Replying to Algebra homework

yea im in algebra, no im not stupid im just in 8th grade, and maybe its just because i cant concentrate right now but can someone help me with this question?

City bus #14 arrives at Grand Street every 10 minutes, starting at 6:00 AM. the dispatcher is settingthe schedule for an additional bus that will arrive at Grand Street every 20 minutes. The dispatcher does not want the two busses to arrive at Grand Street at the same time. Which of these starting times will be best for the additional bus?