First, use division to determine the unit rate of one cookie (sorry if terminology is wrong, but i do know how to do it).
Little town = 9/$8 = $1.125 per cookie
College town = 30/$13 ≈ $2.3076 per cookie
Now we are sure of how much each costs and can determine how much she would have spent at each place.
Little town= $1.125*10= $11.25
College town= $2.3076*10≈ $23.08
Now, subtract little town from college town to figure out the savings.
23.08-11.25= $11.83
Thus, she saved $11.83.