Meredith is addressing envelopes for a charity event. She has finished addressing 30 envelopes and is addressing more envelopes at the rate of 6 envelopes per hour. Meredith's sister, Lily, joined her immediately and started addressing at the rate of 9 envelopes per hour. The number of envelopes addressed by both are shown in the graph below. After how many hours will the number of envelopes addressed by Meredith and Lily be the same?