SOLUTION: How much water needs to be added to 400mL of 77% solUTION to make it 55% solUTION? I know that the answer is 160 mL, but I do not know how to get the answer.

Question 777720: How much water needs to be added to 400mL of 77% solUTION to make it 55% solUTION? I know that the answer is 160 mL, but I do not know how to get the answer.
Answer by josgarithmetic(39617) (Show Source): You can put this solution on YOUR website!
Let v = volume in mL of water to add.
Let P = concentration of solution as percent, 77%.
Let R = desired concentration of solution after mixing the v water, 55%.
Let k = volume of the 77% solution which will be diluted, 400 mL.

SOLVE FOR v.
Substitute the given values to compute v.
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