The total number of pounds of trail mix is 15. The pounds of granola added to the pounds raisins has to equal 15. So, one of the equations is g + r=15.
In total the cost of the granola and the cost of the raisins must be equal to $37.50. Since the granola costs $2 per pound and the raisins cost $4.50 per pound, 2g + 4.5r= 37.5.
