Grass growing prealgebra question

Question

Imagine a new pasture with grass growing on it. Every day after the seeds have germinated, the number of blades of grass in the pasture triples. After 18 days, the entire pasture is completely covered in grass. How many days did it take for the pasture to be one-third covered in grass?

Answer 1

Pick any day. That day has X grass. The grass triples every day. So, the next day has 3X grass. Also, the day before has 1/3X grass.

After 18 days, the entire pasture was covered, so when was it 1/3 covered? The day before, which is after 17 days.