Question

Nikki is planting tomato plants in her garden. She has 20 feet along the back side where she plans to plant as many tomato plants as she can fit. The instructions specify to allow at least 20 inches between the holes where the plants will be planted. Assuming she wants all of the plants to be evenly spaced and that she plans to allow at least 10 inches between the plants on either side of the row and the fence on the sides, determine the number of plants she should buy. Also determine exactly where the plants should be planted to meet the conditions.

I honestly have no idea how to start on this .... 20 feet is 240 inches.. 10" on either side and 20" between plants

If I take 240" - 20" (2 x 10" for what is on the end), that leaves me with 220" for the plants, and that, divided by 20" is 11, so I could say she should buy 11 plants, but...that doesn't take into account how big the holes are.

And then, where to plant the plants....
from the left, at 10", 30", 50", 70", 90", 110", 130", 150", 170", 190", 210", 230"

That seems too easy and not correct?