Solve {l}{5x+20(x+y)=4}{4y+20(x+y)=6}

$$\left\{ \begin{array} { l } { 5 x + 20 ( x + y ) = 4 } \\ { 4 y + 20 ( x + y ) = 6 } \end{array} \right.$$

Solve for x, y

$$x=-\frac{3}{25}=-0.12$$

$$y=\frac{7}{20}=0.35$$

Steps Using Substitution

Consider the first equation. Use the distributive property to multiply 20 by x+y.

$$5x+20x+20y=4$$

Combine 5x and 20x to get 25x.

$$25x+20y=4$$

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

$$4y+20x+20y=6$$

Combine 4y and 20y to get 24y.

$$24y+20x=6$$

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.

$$25x+20y=4,20x+24y=6$$

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

$$25x+20y=4$$

Subtract 20y from both sides of the equation.

$$25x=-20y+4$$

Divide both sides by 25.

$$x=\frac{1}{25}\left(-20y+4\right)$$

Multiply \frac{1}{25} times -20y+4.

$$x=-\frac{4}{5}y+\frac{4}{25}$$

Substitute -\frac{4y}{5}+\frac{4}{25} for x in the other equation, 20x+24y=6.

$$20\left(-\frac{4}{5}y+\frac{4}{25}\right)+24y=6$$

Multiply 20 times -\frac{4y}{5}+\frac{4}{25}.

$$-16y+\frac{16}{5}+24y=6$$

Add -16y to 24y.

$$8y+\frac{16}{5}=6$$

Subtract \frac{16}{5} from both sides of the equation.

$$8y=\frac{14}{5}$$

Divide both sides by 8.

$$y=\frac{7}{20}$$

Substitute \frac{7}{20} for y in x=-\frac{4}{5}y+\frac{4}{25}. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-\frac{4}{5}\times \frac{7}{20}+\frac{4}{25}$$

Multiply -\frac{4}{5} times \frac{7}{20} by multiplying numerator times numerator and denominator times denominator. Then reduce the fraction to lowest terms if possible.

$$x=\frac{-7+4}{25}$$

Add \frac{4}{25} to -\frac{7}{25} by finding a common denominator and adding the numerators. Then reduce the fraction to lowest terms if possible.

$$x=-\frac{3}{25}$$

The system is now solved.

$$x=-\frac{3}{25},y=\frac{7}{20}$$

Steps Using Matrices

Consider the first equation. Use the distributive property to multiply 20 by x+y.

$$5x+20x+20y=4$$

Combine 5x and 20x to get 25x.

$$25x+20y=4$$

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

$$4y+20x+20y=6$$

Combine 4y and 20y to get 24y.

$$24y+20x=6$$

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

$$25x+20y=4,20x+24y=6$$

Write the equations in matrix form.

$$\left(\begin{matrix}25&20\\20&24\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}4\\6\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}25&20\\20&24\end{matrix}\right).

$$inverse(\left(\begin{matrix}25&20\\20&24\end{matrix}\right))\left(\begin{matrix}25&20\\20&24\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}25&20\\20&24\end{matrix}\right))\left(\begin{matrix}4\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}25&20\\20&24\end{matrix}\right))\left(\begin{matrix}4\\6\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}25&20\\20&24\end{matrix}\right))\left(\begin{matrix}4\\6\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{24}{25\times 24-20\times 20}&-\frac{20}{25\times 24-20\times 20}\\-\frac{20}{25\times 24-20\times 20}&\frac{25}{25\times 24-20\times 20}\end{matrix}\right)\left(\begin{matrix}4\\6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{3}{25}&-\frac{1}{10}\\-\frac{1}{10}&\frac{1}{8}\end{matrix}\right)\left(\begin{matrix}4\\6\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{3}{25}\times 4-\frac{1}{10}\times 6\\-\frac{1}{10}\times 4+\frac{1}{8}\times 6\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-\frac{3}{25}\\\frac{7}{20}\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=-\frac{3}{25},y=\frac{7}{20}$$

Steps Using Elimination

Consider the first equation. Use the distributive property to multiply 20 by x+y.

$$5x+20x+20y=4$$

Combine 5x and 20x to get 25x.

$$25x+20y=4$$

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

$$4y+20x+20y=6$$

Combine 4y and 20y to get 24y.

$$24y+20x=6$$

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.

$$25x+20y=4,20x+24y=6$$

To make 25x and 20x equal, multiply all terms on each side of the first equation by 20 and all terms on each side of the second by 25.

$$20\times 25x+20\times 20y=20\times 4,25\times 20x+25\times 24y=25\times 6$$

Simplify.

$$500x+400y=80,500x+600y=150$$

Subtract 500x+600y=150 from 500x+400y=80 by subtracting like terms on each side of the equal sign.

$$500x-500x+400y-600y=80-150$$

Add 500x to -500x. Terms 500x and -500x cancel out, leaving an equation with only one variable that can be solved.

$$400y-600y=80-150$$

Add 400y to -600y.

$$-200y=80-150$$

Add 80 to -150.

$$-200y=-70$$

Divide both sides by -200.

$$y=\frac{7}{20}$$

Substitute \frac{7}{20} for y in 20x+24y=6. Because the resulting equation contains only one variable, you can solve for x directly.

$$20x+24\times \frac{7}{20}=6$$

Multiply 24 times \frac{7}{20}.

$$20x+\frac{42}{5}=6$$

Subtract \frac{42}{5} from both sides of the equation.

$$20x=-\frac{12}{5}$$

Divide both sides by 20.

$$x=-\frac{3}{25}$$

The system is now solved.

$$x=-\frac{3}{25},y=\frac{7}{20}$$