Question 450144
Let x = the number of lbs of $20 coffee
Since there are 50 lbs of $50 coffee, the final mixture will contain x+50 lbs
We want the mixture to sell for $40 per pound, so we can write
40(x+50) = 50*50 + 20x
Solve for x:
40x + 2000 = 2500 + 20x
20x = 500 
x = 25
Therefore, she should use 25 lbs of $20 coffee