See page 57 in the Manhattan statistics book....
No formulas needed. Look at the mix. It has a cost of $22. What is the differential for the First grade tobacco? +$8. What is the differential for the Second Grade Tobacco? -$2.
In order to get the +8 and -2 to cancel out, we'd need to multiply the -2 by 4.
Ex: (1)(8) - (4)(-2) = 0
Therefore, the ratio of the Second grade to First grade is 4:1
So for every 4 parts second grade there is 1 part first grade.
The question asks for the percentage of second grade in the blend. Well we know the blend has 4 parts second and 1 part first grade, so 5 parts total. We also know that second grade is 4 of the total 5 parts. So, 4/(4+1) or 4/5 = 8/10 = 80%