Answer:
 We should add 2.4 quarts of pure orange juice to 3.6 quarts 10% orange juice to make 6 quarts of 40% orange juice.
Step-by-step explanation:
 Let p represent number of quarts of pure orange juice and q represent number of quarts of 10% orange juice.
 We have total quarts required = 6
 So, p+q =6 --------------------equation 1
 The orange content of 6 quarts = 40 % = 6 x 40/100 = 2.4 quarts.
Orange content in p pure juice = p quarts.
Orange content in q, 10% juice = q x 10/100 = 0.1 q quarts
So we have, p + 0.1 q = 2.4 --------------------equation 2
Equation 1 - Equation 2 gives
 0.9 q = 6 -2.4
 q = 3.6 quarts
 p + 3.6 = 6
 p = 2.4 quarts
So we should add 2.4 quarts of pure orange juice to 3.6 quarts 10% orange juice to make 6 quarts of 40% orange juice.