Question 944809
The ratios of A, B, and C need to be made clear through symbolism.


Using A, B, C, for their corresponding quantities,
{{{system(B=2A,B=-3+C)}}}


Try to account for the cost of the mixture.
{{{0.6A+0.4B+0.8C=11.40}}}



Recheck the system of the ratios among A, B, and C.  The system can be rearranged to show the quantities as functions of B.
{{{system(A=B/2,C=B+3)}}}.
Substitute these for A and C in the cost equation and have an equation in just the one variable, B.


....
You do that.
Can you finish?