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 Question from Adom, a student: I have 500mls of 90% conc. of an acid. Now I need to prepare 20% concentration from this (say 100mls of this). How much of the stock solution do I take and how much water to add? Thanks!

You need 100 ml of a 20% solution so you need 20% of the 100 ml to be undiluted acid, that is $0.20 \times 100 = 20 \mbox{ ml.}$ Suppose $x$ ml of the 90% solution is used to get the 20 ml of undiluted acid you need. That is 90% of $x$ is 20 ml, or $0.90 \times x = 20 \mbox{ ml.}$
What is $x?$