25 ml of solution is at 50%. That means that it contains 50% of the solute e.g.

let us assume it is alcohol.

I.e. in a 50% solution of 25 ml, volume of alcohol = 25*50% =  25 * 0.5 = 12.5 ml

Now the question, if you had the same volume of alcohol, i.e. 12.5 ml, what is 

the total volume of the mixture to make it a 30% solution.

If the 30% solution mixture has a volume of x ml, and 12.5 ml of it is alcohol,

then it means that:

x * 30% = x * 0.3 = 12.5

x = 12.5/0.3 = 41.67 ml approximately.

So to get the same amount of alcohol, you have to use 41.67 ml of a 30% solution.

Hope you got it :)

 

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