Question 712581
Your beginning seems good.  The given information can let us set two equations: one for cost (or price?) and one for pounds.


For price of mix, {{{6R+5L+4B=5.50}}}, dollars per pound.

For poundage sum, {{{R+L+B=10}}}, pounds.


The information so far given is enough to make the question open, with more than one solution result.  If you are expected to give one fundamental solution, then you must have additional information.  You only have two equations but THREE unknown variables.


The current system allows you to eliminate and solve for any two of the variables, but their solutions must be in terms of the remaining variable.  So which variable you want as free, is your decision.  How do you want to continue next?