20 cm³ of pure alcohol is added to 10 cm of a 10% alcohol solution to strengthen it to a 70% solution.
Let x represent the volume of pure alcohol (100%) in cm³ needed to be added. Hence:
x * 100% + 10 cm³ * 10% = 70% * (x + 10 cm³)
x * 1 + 10 * 0.1 = 0.7 * (x + 10)
x + 1 = 0.7x + 7
x - 0.7x = 7 - 1
0.3x = 6
x = 20 cm³
Therefore 20 cm³ of pure alcohol is added to 10 cm of a 10% alcohol solution to strengthen it to a 70% solution.
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