Question 950649
The density or at least specific gravity is needed.  The concentration 85% should mean "weight per weight".  "ltr"  appears to mean "liters", and this is volume.  <s>No density - no solving.</s>


Continuing staying all with mass  ("weight"),


10*liters of water,


{{{10*liters*1000(ml/liter)*1(gram/ml)(1/1000)(kg/grams)}}}


{{{10*kilograms}}}, the starting mass of water.


An old handbook shows listed, 1.689 for specific gravity of 85% phosphoric acid; and temperature may be ignored just for convenience.


Let p be mass in kilograms of the 85% acid to add.

{{{highlight_green(85*p/(p+10)=3)}}}

Solve for p.