Question 1078955
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Let x be the amount (the volume, in liters) of the 25% orange juice to mix.


Then the required volume of the 5% orange juice is (20-x) liters.


The amount of the pure orange juice in the mixture is 

0.25x + 0.05(20-x)

(motice that 0.25 is 25%  and 0.05 is 5%).


This amount of the orange juice must be equal to 0.21*20 = 4.2 liters, as the condition requires.

So, you get this equation

0.25x + 0.05*(20-x) = 4.2.


Simplify and solve for x:

0.25x + 1.0 - 0.05x = 4.2,

0.2x = 4.2 - 1.0,

0.2x = 3.2

x = {{{3.2/0.2}}} = 16.


<U>Answer</U>.  16 liters of the 25% juice and 20-16 = 4 liters of the 5% juice must be mixed.

<U>Check</U>.   0.25*16 + 0.05*4 = 4 + 0.2 = 4.2 liters of the pure juice.

         Then the percentage of the pure juice in 20 liters of liquid is {{{4.2/20}}} = 0.21 = 21%. Correct !!
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