Question 1011242
x for cheap
y for expensive
{{{system(x+y=5,(3.5x+4y)=18)}}}------one equation accounts for material quantities and other accounts for cost of the mixture.


You can use some Elimination if try the system this way:
{{{system(4x+4y=20,3.5x+4y=18)}}}


That gives you  {{{x/2=2}}},
{{{highlight(x=4)}}}.