Question
Solve the following simultaneous equation.
99x + 101 y = 499; 101x + 99y = 501

Answer

$99 x+101 y=499 \ldots(I) $
$101 x+99 y=501 \ldots(II)$
Adding both the Equations
$99 x+101 y =499 $
$101 x+99 y =501$
$--------$
$200 x+200 y  =1000$
Dividing both sides by 200
$x+y=5\dots(III)$
Subtract equation (I) and (II)
$\begin{array}{c}99 x+101 y=499 \\-101 x-99 y=-501 \\\hline-2 x+2 y=-2\end{array}$
Divide both sides by ( -2 )
$x-y=1 \ldots( IV )$
Equating Eq. (III) and (IV)
$x+y=5 $
${x-y=1}$
$----$
${2 x=6} $
$x=\frac{6}{2} $
$x=3$
Substituting $x=3$ in Eq. III
$3+y=5 $
$y=5-3$
$y=2$
$\therefore$ solution is $(x, y)=(3,2)$

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