d = number of days (some positive whole number, or 0)
5*d = amount of miles ran after d days where you run 5 miles per day
100 - 5*d = number of miles left to run if the goal is to run 100 miles total
For example, let's say that d = 2 meaning that 2 days have gone by. If you run 5 miles per day, then 5*d = 5*2 = 10 miles have been run so far leaving 100-10 = 90 left over.
If we plugged d = 2 into the expression we found, then,
100 - 5*d = 100 - 5*2 = 100 - 10 = 90
and we get the same result.
You have the correct answer. It is choice A) 100 - 5d. Nice work on getting the correct answer.
