There are 150 marigold plants in a back yard. Each month, the number of marigold plants decreases by 15 plants. There are 100 sunflower plants in the back yard. Each month, 10 sunflower plants are added. Part A: Write functions to represent the number of marigold plants and the number of sunflower plants in the back yard throughout the months.

Part B: How many marigold plants are in the back yard after 6 months? How many sunflower plants are in the back yard after the same number of months?

Part C: After approximately how many months is the number of marigold plants and the number of sunflower plants the same? Justify your answer mathematically.