Question

Alex usually runs for 21 hours a week,

Answer 1

Alex runs 21 hours a week.
Since, a week has 7 days, he runs, 21 hours in 7 days.
Now,
In 7 days, Alex runs 21 hours
In 1 day, Alex runs 21 / 7 hours
                            = 3 hours
In 3 days, Alex runs 3 * 3 hours
                           = 9 hours.
 
So, If he is unable to run for 3 days, he loses 9 hours of his training time.