Question 1125449
...,  if 5 kg of brand A is used how many kg of brand A should be used to create a mixture with average cost of,...


Decide which.  Not the same unknown and known.


Until you decide if it is given 5 pounds of Brand A or 5 pounds of Brand B , A is amount of Brand A and B is amount of Brand B.


Accounting for cost,  {{{5A+8B=6(A+B)}}}.

Either A is 5, and solve for B; Or B is 5, and solve for A.