Question 1088823
Assuming percents to be mass per mass, then you need  these:
<pre>
85% ----------- 1.689 grams/ml density
15% ----------- 1.0824 grams/ml density
</pre>


w, mass of water to add

{{{(1000*1.689*85)/(w+1000*1.689)=15}}}
Solve this to get the MASS of water to use, in grams... and then you can convert into its volume if you wish...




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online ref,  https://wissen.science-and-fun.de/chemistry/chemistry/density-tables/density-of-phosphoric-acid/