Alligation calculation missing information

how many milliters of a 50% stock solution is needed to prepare 150 ml of a 30% solution? dont understand.

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Reply (by Keith)

I think what is confusing about this is that you dont see a lower number concentration to mix with the 50%.

So, what you need to know is that you Always have 0% to work with.

So now, work this (Alligation) problem with 50% and 0%, working to make 30% solution.