Question 31486
Hello!
The speed is measured as {{{distance/time}}}. Let's call X to the time he takes to drive 20 miles in the side roads (which is the same as the time he takes to drive 15 miles on the freeway). The nwe have:

Speed on side road = {{{20/X}}}
Speed on freeway = {{{15/X}}}

But we also know that his speed on the side road is 9 mph faster than the one on the freeway. So we get the equation:

{{{20/X = 15/X + 9}}}

 Multiplying both sides by X in order to get rid of the X in the denominator:

{{{20 = 15 + 9X}}}
{{{5 = 9X}}}
{{{X = 5/9}}}

We have that he takes 5/9 hour in order to drive 20 miles on the side road. Therefore, his speed must be:
{{{20/(5/9) = 36}}}

His speed on the side road is 36 mph


I hope this helps!
Get more answers at <a href=http://www.onlinemathanswers.com>Online Math Answers.com</a>!