Solve {l}{x+y=100}{30x+20y=72000/30}

$$\left\{ \begin{array} { l } { x + y = 100 } \\ { 30 x + 20 y = \frac { 72000 } { 30 } } \end{array} \right.$$

Solve for x, y

$$x=40$$

$$y=60$$

Steps Using Substitution

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.

$$x+y=100,30x+20y=2400$$

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

$$x+y=100$$

Subtract y from both sides of the equation.

$$x=-y+100$$

Substitute -y+100 for x in the other equation, 30x+20y=2400.

$$30\left(-y+100\right)+20y=2400$$

Multiply 30 times -y+100.

$$-30y+3000+20y=2400$$

Add -30y to 20y.

$$-10y+3000=2400$$

Subtract 3000 from both sides of the equation.

$$-10y=-600$$

Divide both sides by -10.

$$y=60$$

Substitute 60 for y in x=-y+100. Because the resulting equation contains only one variable, you can solve for x directly.

$$x=-60+100$$

Add 100 to -60.

$$x=40$$

The system is now solved.

$$x=40,y=60$$

Steps Using Matrices

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

$$x+y=100,30x+20y=2400$$

Write the equations in matrix form.

$$\left(\begin{matrix}1&1\\30&20\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}100\\2400\end{matrix}\right)$$

Left multiply the equation by the inverse matrix of \left(\begin{matrix}1&1\\30&20\end{matrix}\right).

$$inverse(\left(\begin{matrix}1&1\\30&20\end{matrix}\right))\left(\begin{matrix}1&1\\30&20\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\30&20\end{matrix}\right))\left(\begin{matrix}100\\2400\end{matrix}\right)$$

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

$$\left(\begin{matrix}1&0\\0&1\end{matrix}\right)\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\30&20\end{matrix}\right))\left(\begin{matrix}100\\2400\end{matrix}\right)$$

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

$$\left(\begin{matrix}x\\y\end{matrix}\right)=inverse(\left(\begin{matrix}1&1\\30&20\end{matrix}\right))\left(\begin{matrix}100\\2400\end{matrix}\right)$$

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.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}\frac{20}{20-30}&-\frac{1}{20-30}\\-\frac{30}{20-30}&\frac{1}{20-30}\end{matrix}\right)\left(\begin{matrix}100\\2400\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-2&\frac{1}{10}\\3&-\frac{1}{10}\end{matrix}\right)\left(\begin{matrix}100\\2400\end{matrix}\right)$$

Multiply the matrices.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}-2\times 100+\frac{1}{10}\times 2400\\3\times 100-\frac{1}{10}\times 2400\end{matrix}\right)$$

Do the arithmetic.

$$\left(\begin{matrix}x\\y\end{matrix}\right)=\left(\begin{matrix}40\\60\end{matrix}\right)$$

Extract the matrix elements x and y.

$$x=40,y=60$$

Steps Using Elimination

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.

$$x+y=100,30x+20y=2400$$

To make x and 30x equal, multiply all terms on each side of the first equation by 30 and all terms on each side of the second by 1.

$$30x+30y=30\times 100,30x+20y=2400$$

Simplify.

$$30x+30y=3000,30x+20y=2400$$

Subtract 30x+20y=2400 from 30x+30y=3000 by subtracting like terms on each side of the equal sign.

$$30x-30x+30y-20y=3000-2400$$

Add 30x to -30x. Terms 30x and -30x cancel out, leaving an equation with only one variable that can be solved.

$$30y-20y=3000-2400$$

Add 30y to -20y.

$$10y=3000-2400$$

Add 3000 to -2400.

$$10y=600$$

Divide both sides by 10.

$$y=60$$

Substitute 60 for y in 30x+20y=2400. Because the resulting equation contains only one variable, you can solve for x directly.

$$30x+20\times 60=2400$$

Multiply 20 times 60.

$$30x+1200=2400$$

Subtract 1200 from both sides of the equation.

$$30x=1200$$

Divide both sides by 30.

$$x=40$$

The system is now solved.

$$x=40,y=60$$