Ella bought some key chains and spent a total of $40. Each key chain cost the same whole-dollar amount. She bought between 9 and 13 key chains. How many key chains did Ella buy?

answer: evaluate the objective function at each vertex to determine which - and -values, if any, maximize or minimize the function. example: find the minimum value and maximum value of the objective function f ( x , y ) = 4 x + 5 y , subject to the following constraints