The lily is watered on days 4, 8, 12, 16, 20, 24, 28, 32, 36, etc The violet is watered on days: 7, 14, 21, 28, 35, etc
Therefore the day when you water both is day 28 or 4 x 7 = 28
28 days is also four weeks so you can set a date four weeks hence when you know that you will be watering both on the same day.
You will first water both the lily and the African violet on Day 28. After this, you will continue to water them together every 28 days, which will occur on Days 56, 84, 112, and so on.
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