The shrub had already grown to 20 inches in height when it was planted. Thus, assuming that it was cared for, it continued to grow, to 32 inches. How many weeks did this require, given that the growth rate is 1/2 inch per week?
Let's not mix "weeks" and "days," at least not without a conversion factor.
Only the 2nd possible answer fits this situation. Shane planted a shrub that was already 20 inches tall (this is the y-intercept). The shrub grows at the rate of 1/2 inch per week, until it reaches a height of 32 inches.
height(w) = 20 inches + (1/2 inch per week) (# of weeks)
32 in = 20 in + (1/2 inch per week)(# of weeks)
12 in = (1/2 in/wk) (# of weeks)
12 in
-------------- = 24 weeks
1/2 in/wk
