A coffee merchant combines coffee that costs $7 per pound with coffee that costs $3.60 per pound. How many pounds of each should be used to make 26 lb of a blend costing $6.15 per pound?

First you want to set up two equations. Then substitute one into the other to solve:

Let x be pounds of $7 coffee and y be pounds of $3.60 coffee which equals the total pounds of coffee:

x+y=26

The total cost of blended coffee is calculated by adding the total cost of the two different coffees together:

7.00(x)+360(y)=6.15(26)

To solve for one variables use the substitution method:

x+y=26

x=26-y

7(26-y)+3.6y=159.90

182-7y+3.6y=159.90

3.4y=-22.1

y=6.5lb

x=26-6.5=19.5lb

So it takes 19.5lb of the $7/lb coffee and 6.5lb of the $3.50/lb coffee to make 26lb of the blended coffee at $6.15/lb.