Question 159493
X= number of Liters of pure alcohol needed 

Convert the % to decimal format ie 21%=.21
Pure alcohol is 100% decimal format is 1

The algebraic equation for this is:

X+7(.21)=.447(7+X)

The total volume on both sides of the equation must be the same.

Use the distributive property to get

X+1.47=3.129+.447X

Subtract .447 from both sides
Subtract 1.47 from both sides

0.553X=1.654
X=3 Liters of pure alcohol
To confirm plug in 3 for the variable X in the algebraic equation.