Question 1007013
{{{system((30x+70y)/90=62,x+y=90)}}}


simpler,
{{{system((3x+7y)/9=62,x+y=90)}}}
Not tricky.  Linear system.


You could also choose to do in just one single variable as demonstrated here:
<a href="https://www.youtube.com/watch?v=-REjxT9KCzQ">just one variable to solve two-part percent mixture</a>