Alex usually runs for 21 hours a week, training for a marathon. if he is unable to run for 3 days, describe how to find out how many hours of training time he loses, and write the appropriate integer to describe how it affects his time

Alex usually runs for 21 hours a week.  We know that there are 7 days in a week.  21/7 = 3, which means that Alex runs for 3 hours each day.

We are trying to figure out how much time he loses if he is unable to run for three days.  3 * 3 = 9.  This means he loses 9 hours of training while he is unable to run.

Assuming he runs the same number of hours per day, then he runs 3 hours per day (21/7). 
If he runs 3 hours a day, then in 3 days he would have ran 9 hours (3*3). 

He loses 9 hours of training if he doesn't run for 3 days.