Jon P. answered 07/16/15
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As you stated, this is a linear programming problem.
First let's choose the variables... Let x = the number of language courses, let y = the number of math courses, and let z = the number of physics courses.
The objective function to be minimized is the total cost. Based on the prices given, that's 15x + 12y + 18z.
Now let's use the other information to create the constraints.
"sydney the turtle must take at least 35 classes"
x + y + z ≥ 35
"and accumulate a minimum of 130 credits"
3x + 4y + 5z ≥ 130
"a minimum of 5 each type of course be taken"
x ≥ 5
y ≥ 5
z ≥ 5
So to summarize...
The objective function is 15x + 12y + 18z, and it is to be minimized.
There are 5 constraints:
x + y + z ≥ 35
3x + 4y + 5z ≥ 130
x ≥ 5
y ≥ 5
z ≥ 5