Question 984107
Both cans have only "yellow" and "blue".  This seems like a description of the typical simple two-part mixture type.


First Can, 60% Yellow
Second Can, 30% Yellow

An attempt at doing this solution in just one single variable:


Let v be how much gallons of the first can of 60% yellow.
Then 4-v is gallons of the second can at 30% yellow.


{{{volumeYellow/volumePaint=FractionYellow}}}


{{{(60*v+30*(4-v))/4=50}}}


Immediately simplify if doing with the given values already present.
{{{(6v+3(4-v))/4=5}}}


{{{6v+12-3v=5*4}}}

{{{3v+12=20}}}

{{{3v=8}}}

{{{highlight(v=8/3=2&2/3)}}}, gallons of the 60% yellow paint.
This means {{{1&1/3}}} of the 30% yellow paint.