Answer: Dark Chocolate = 2.5 lbs, Milk Chocolate = 0.5 lbs
Explanation:
 Qty % Qty * %
Dark Chocolate: x 0.25 0.25(x) = 0.25x
Milk Chocolate:  3 - x  0.75 0.75(3 - x) = 2.25 - 0.75x
 Mixture: 3 100% = 1
Part A: 0.25x + 2.25 - 0.75x = 1
 2.25 - 0.50x = 1
  -2.25   -2.25 
 -0.50x = -1.25
  ÷ -0.50  ÷ -0.50 
 x = 2.5
Milk Chocolate: 3 - x = 3 - 2.5 = 0.5