Algebra: must make a chart
$/lb * # of lbs = total
Oak 3.4 x 3.4x
Pine 2.6 40-x 104-2.6x
Mixture 2.8 40 112
Now you can set up your equation ( assuming you know how to do equations 'cuz I figure you're in algebra)
3.4x + 104 - 2.6x = 112
.8x + 104 = 112
.8x = 8
10 pounds of oak
30 pounds of pine