Answer:
Casandra will run 8 miles on the 9th Day.
Explanation:
Given:
Casandra on first day of training she runs 4 miles
So we can say that;

Also Given:
each day after that she adds on 0.5 mile.
So we can say that;
Common difference

We need to find on what day she will run 8 miles.
So we can say that;

Let the number of the day be denoted by 'n'
Solution:
Now By using the formula of Arithmetic Progression we get;

Now substituting the values we get;

Now by using distributive property we get;

Now subtracting both side by 3.5 we get;

Dividing both side by 0.5 we get;

Hence Casandra will run 8 miles on the 9th Day.