Solving Mixture word problems? I am having very difficult times in understanding the following and related mixture problems.Can anybody guide me the easy and nice trick that is useful in understanding,visualizing and solving these types of problems? following are few of those.


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*How many liters of a 70% alcohol solution must be added to 50 liters of a 40% alcohol solution to produce a 50% alcohol solution?

*How many ounces of pure water must be added to 50 ounces of a 15% saline solution to make a saline solution that is 10% salt?

*Find the selling price per pound of a coffee mixture made from 8 pounds of coffee that sells for \$9.20 per pound and 12 pounds of coffee that costs \$5.50 per pound?

*How many pounds of lima beans that cost $0.90 per pound must be mixed with 16 pounds of corn that costs \$0.50 per pound to make a mixture of vegetables that costs \$0.65 per pound?

*Two hundred liters of a punch that contains 35% fruit juice is mixed with 300 liters (L) of another punch. The resulting fruit punch is 20% fruit juice. Find the percent of fruit juice in the 300 liters of punch?

*Ten grams of sugar are added to a 40-g serving of a breakfast cereal that is 30% sugar. What is the percent concentration of sugar in the resulting mixture?

Let's consider $1$st problem.
Problem:


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*How many liters of a 70% alcohol solution must be added to 50 liters of a 40% alcohol solution to produce a 50% alcohol solution?


"Easy and nice trick" :)
Let's consider liquids separately: 
 - alcohol; 
 - water.
 
Then image:

Then one can write equation for each liquid:
for alcohol: 
$$\qquad 0.7 \cdot x + 0.4 \cdot 50 = 0.5 \cdot (x+50);\tag{1}$$
or for water: 
$$\qquad 0.3 \cdot x + 0.6 \cdot 50 = 0.5 \cdot (x+50).\tag{2}$$
Then solve $(1)$ (or $(2)$ ) :
$$
0.2\cdot x=5;
$$
$$
x=25 \mbox{ (liters)}.
$$
