Answer:
Therefore you have to run every mile in ~ 8 minutes.
Explanation:
10 miles - 3 miles = 7 miles left. 
 
1 hour 20 minutes - 24.7 minutes= 80 minutes - 24.7 minutes 
 = 55.3 minutes left 
 
Therefore, you have 55.3 minutes left to run 7 miles. But since you want to finish in LESS than 1 hour and 20 minutes, subtract the .3. 
 
So: 
 
7 miles = 55 minutes 
1 mile x minutes 
 
x minutes= 55 x 1 ÷ 7 miles 
x minutes = 7.85 
x minutes = ~ 8 minutes 
 
Therefore you have to run every mile in ~ 8 minutes. 
 
Hope this helps!