This can be solved using a system of equations.
[tex] \left \{ {{x+y=80} \atop {x-4y=10} \right.
[/tex]
Subtract y from both sides.
[tex]x=80-y[/tex]
Substitute:
[tex]80-y-4y=80-5y=10[/tex]
Subtract 80 from both sides:
[tex]5y=70[/tex]
Divide both sides by 5:
[tex]y= \frac{70}{5} =14[/tex]
Substitute.
[tex]x=80-14=66[/tex]
