Answer:
Speed of Sharon on country roads = 50 mph
Step-by-step explanation:
Let the speed of Sharon on interstate roads was = a mph
And the speed on country roads = b mph
Time taken by Sharon to travel 390 miles on interstate = 
=
hours
Time taken by Sharon to travel 150 miles on country roads =
hours
"Total time for the trip = 9 hours"
--------(1)
"Her speed on interstate was 15 mph more than on country roads".
a = b + 15 ---------(2)
By substituting the value of a from equation (2) to equation (1),


180b + 750 = 3b(b + 15)
180b + 750 = 3b² + 45b
3b² + 45b - 180b - 750 = 0
3b² - 135b - 750 = 0
b² - 45b - 250 = 0
b² - 50b + 5b - 250 = 0
b(b - 50) + 5(b - 50) = 0
(b + 5)(b - 50) = 0
b = -5, 50
But the speed can't be negative,
Therefore, b = 50 mph
Speed of Sharon on country roads was 50 mph.