Curtis kept track of the number of miles he ran each day for 10 days. by day 4, he had already run 9 miles. how many days did it take curtis to run 21 miles? 9 days 25 days 30 days 7 days

  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.

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.

i'm assuming that it's c.)   volcanoes built the mountains and earthquakes tore them down.