Question 755890
Let v = volume liters of the 70% acid to use.


Resulting volume of solution is v+14 liters.


{{{(70v+20*14)/(v+14)=35}}}
Solve for v.


{{{70v+20*14=35v+35*14}}}
{{{(70-35)v=35*14-20*14=(35-20)*14}}}
You finish!