How much coffee costing $6.00 per pound should be mixed with 3 pounds of coffee costing $4.00 per pound to create a mixture costing $4.50 per pound?

We'll call the unknown quantity x 
6x-total cost of 6 dollar coffee
(3)(4) -total cost of 4 dollar coffee 

so our working equation is 

6x+12=(x+3)(4.5) 

which when solved gives us x=1 pound 

:)