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frika

You have 640 ml of a 11% solution and want to know how much ml of pure acid there is.

Then

640 ml - 100%,

x ml - 11%.

Use proportion:

[tex]\dfrac{640}{x}=\dfrac{100}{11},\\ \\x=\dfrac{640\cdot 11}{100}=70.4[/tex] ml.

Answer: 70.4 ml