Let x represent the number of 47-cent stamps that Mylee bought, and y the number of 34-cent stamps. Using the given information you can set the following system of equations:
[tex]\begin{gathered} 0.47x+0.34y=21.55, \\ x+y=50. \end{gathered}[/tex]Multiplying the second equation by 0.47 and subtracting it from the first equation, you get:
[tex]0.47x-0.47x+0.34y-0.47y=21.55-23.5.[/tex]Solving the above equation for y, you get:
[tex]\begin{gathered} -0.13y=-1.95, \\ y=\frac{1.95}{0.13}, \\ y=15. \end{gathered}[/tex]Using the second equation of the system you get:
[tex]\begin{gathered} x+15=50, \\ x=50-15, \\ x=35. \end{gathered}[/tex]