Question 897310
...solution t,
What?


Let v= volume of 90% acid to use.

{{{(20*60+v*90)/(20+v)=t}}}
t would mean, percent acid of the mixture, and you must know what this is.  Solving for v is then possible.


{{{20*60+90v=t(v+20)}}}
{{{1200+90v=t*v+20t}}}
{{{90v-tv=20t-1200}}}
{{{v(90-t)=20t-1200}}}
{{{highlight_green(v=(20t-1200)/(90-t))}}}