Solve {l}{x+y=18}{31x=2(20+y)}

$$\left\{ \begin{array} { l } { x + y = 18 } \\ { 31 x = 2 ( 20 + y ) } \end{array} \right.$$

Solve for x, y

$$x = \frac{76}{33} = 2\frac{10}{33} \approx 2.303030303$$

$$y = \frac{518}{33} = 15\frac{23}{33} \approx 15.696969697$$

Steps Using Substitution

Consider the second equation. Use the distributive property to multiply 2 by 20+y.

$$31x=40+2y$$

Subtract 2y from both sides.

$$31x-2y=40$$

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.

$$x+y=18,31x-2y=40$$

Choose one of the equations and solve it for x by isolating x on the left hand side of the equal sign.

$$x+y=18$$

Subtract y from both sides of the equation.

$$x=-y+18$$

Substitute -y+18 for x in the other equation, 31x-2y=40.

$$31\left(-y+18\right)-2y=40$$

Multiply 31 times -y+18.

$$-31y+558-2y=40$$

Add -31y to -2y.

$$-33y+558=40$$

Subtract 558 from both sides of the equation.

$$-33y=-518$$

Divide both sides by -33.

$$y=\frac{518}{33}$$

Substitute \frac{518}{33} for y in x=-y+18. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{518}{33}+18$$

Add 18 to -\frac{518}{33}.

$$x=\frac{76}{33}$$

The system is now solved.

$$x=\frac{76}{33},y=\frac{518}{33}$$

Steps Using Matrices

Consider the second equation. Use the distributive property to multiply 2 by 20+y.

$$31x=40+2y$$

Subtract 2y from both sides.

$$31x-2y=40$$

Put the equations in standard form and then use matrices to solve the system of equations.

$$x+y=18,31x-2y=40$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&1\\31&-2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}18\\40\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&1\\31&-2\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&1\\31&-2\end{matrix}\right))\left(\begin{matrix}1&1\\31&-2\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\31&-2\end{matrix}\right))\left(\begin{matrix}18\\40\end{matrix}\right)$$

The product of a matrix and its inverse is the identity matrix.

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\31&-2\end{matrix}\right))\left(\begin{matrix}18\\40\end{matrix}\right)$$

Multiply the matrices on the left hand side of the equal sign.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\31&-2\end{matrix}\right))\left(\begin{matrix}18\\40\end{matrix}\right)$$

For the 2\times 2 matrix \left(\begin{matrix}a&b\\c&d\end{matrix}\right), the inverse matrix is \left(\begin{matrix}\frac{d}{ad-bc}&\frac{-b}{ad-bc}\\\frac{-c}{ad-bc}&\frac{a}{ad-bc}\end{matrix}\right), so the matrix equation can be rewritten as a matrix multiplication problem.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{2}{-2-31}&-\frac{1}{-2-31}\\-\frac{31}{-2-31}&\frac{1}{-2-31}\end{matrix}\right)\left(\begin{matrix}18\\40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{33}&\frac{1}{33}\\\frac{31}{33}&-\frac{1}{33}\end{matrix}\right)\left(\begin{matrix}18\\40\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{2}{33}\times 18+\frac{1}{33}\times 40\\\frac{31}{33}\times 18-\frac{1}{33}\times 40\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{76}{33}\\\frac{518}{33}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=\frac{76}{33},y=\frac{518}{33}$$

Steps Using Elimination

Consider the second equation. Use the distributive property to multiply 2 by 20+y.

$$31x=40+2y$$

Subtract 2y from both sides.

$$31x-2y=40$$

In order to solve by elimination, coefficients of one of the variables must be the same in both equations so that the variable will cancel out when one equation is subtracted from the other.

$$x+y=18,31x-2y=40$$

To make x and 31x equal, multiply all terms on each side of the first equation by 31 and all terms on each side of the second by 1.

$$31x+31y=31\times 18,31x-2y=40$$

Simplify.

$$31x+31y=558,31x-2y=40$$

Subtract 31x-2y=40 from 31x+31y=558 by subtracting like terms on each side of the equal sign.

$$31x-31x+31y+2y=558-40$$

Add 31x to -31x. Terms 31x and -31x cancel out, leaving an equation with only one variable that can be solved.

$$31y+2y=558-40$$

Add 31y to 2y.

$$33y=558-40$$

Add 558 to -40.

$$33y=518$$

Divide both sides by 33.

$$y=\frac{518}{33}$$

Substitute \frac{518}{33} for y in 31x-2y=40. Because the resulting equation contains only one variable, you can solve for x directly.

$$31x-2\times \frac{518}{33}=40$$

Multiply -2 times \frac{518}{33}.

$$31x-\frac{1036}{33}=40$$

Add \frac{1036}{33} to both sides of the equation.

$$31x=\frac{2356}{33}$$

Divide both sides by 31.

$$x=\frac{76}{33}$$

The system is now solved.

$$x=\frac{76}{33},y=\frac{518}{33}$$