\left\{ \begin{array} { l } { x + 2 y = 20 } \\ { y + 204 = x 20 } \end{array} \right.
Solve for x, y
x=428-2x_{20}
y=x_{20}-204
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y+204=x_{20},2y+x=20
To solve a pair of equations using substitution, first solve one of the equations for one of the variables. Then substitute the result for that variable in the other equation.
y+204=x_{20}
Pick one of the two equations which is more simple to solve for y by isolating y on the left hand side of the equal sign.
y=x_{20}-204
Subtract 204 from both sides of the equation.
2\left(x_{20}-204\right)+x=20
Substitute -204+x_{20} for y in the other equation, 2y+x=20.
2x_{20}-408+x=20
Multiply 2 times -204+x_{20}.
x=428-2x_{20}
Subtract -408+2x_{20} from both sides of the equation.
y=x_{20}-204,x=428-2x_{20}
The system is now solved.
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