Answer:
7 days
Step-by-step explanation:
We assume that "by day 4" means "at the end of day 3." Since Curtis had run 9 miles in 3 days, his average rate is 9/3 = 3 miles per day.
It will take Curtis ...
(21 mi)/(3 mi/day) = 7 days
to run 21 miles at that rate.
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Comment on the question
There is nothing in the problem statement to suggest that the number of miles run each day is the same. A different assumption than the one we have made will give a different answer.
