Several months ago, Deven bought two plants. The two plants had the same height at the time when they were purchased. Ever since then, Plant A grew \(20\) centimeters every month. Plant B grew 1 centimeter in the first month after its purchase, 2 centimeters in the following month, and generally grew n centimeters in the nth month after its purchase. Deven finds out today that the two plants have the same height again. How many months ago did he buy the two plants?