Question 915702
The method here makes sense only if the percents are volume per volume; otherwise, other details might be necessary.


u, volume of 50%
v, volume of 75%


{{{(acid)/(mixture)=fraction}}}
The fraction could be used as a percentage.
{{{(50u+75v)/40=60}}}


{{{50u+75v=2400}}}
{{{10u+15v=480}}}
{{{2u+3v=96}}}


{{{u+v=40}}}, using just sum of the material volume quantities.


Linear system of equations, {{{highlight_green(system(2u+3v=96,u+v=40))}}}