A group of volunteer gardeners are organizing plants. On Monday, they arrange the plants in rows of 15. They find they can arrange the plants in different numbers of equal rows each day, continuing this for 6 days in total (including the arrangement with 1 plant per row and all plants in one row). On the 7th day, they cannot find a new way to form equal rows. What is the smallest possible number of plants?
30
32
45
60
75
