



 
Hi Adom, 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 $x?$ Penny  


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