You can solve the first system of equations by substitution or by manipulating the equations and adding them.
If you multiply both sides of the first equation by -10, you get -4x - y = -2.8. Adding this to the second equation gives you -3x = 22.2, and you can solve the system from there.
For the second system, if the ratio of A to B is 5 to 8, then (according to your setup) x/y = 5/8, or x = 5y/8. You can substitute this value for x into the first equation in order to solve for y and then solve the system.