The Mckinney family has a pond in their yard with water lillies. The area of the lilies' leaves cover twice as much of the pond as they did 35 days ago. The leaves will completely cover the pond in 80 days if they continue to grow unchecked. The McKinneys want to cut back the lilies when they have covered at most half the area of the pond. When is this growth expected to happen?

I don't really understand how to set up this problem. Could someone help?

Btw the answer is 45 days. I just don't know how they got this lol