Solve the following pair of linear equations using a graph: $x+y=5$ and $3x+3y=15$.

(N/A) Here,dividing each term of $3x+3y=15$ by $3$,we get the equation $x+y=5$.
Thus,both the equations of the pair are identical.
Hence,we say that both the lines are the same.
So,they are coincident. It means that there are infinitely many solutions. To draw the graph,we make the following table:
$x+y=5$
$\therefore y=5-x$
For $x=0, y=5-0=5$
For $x=5, y=5-5=0$
and
$3x+3y=15$
$\therefore 3y=15-3x$
$\therefore y=\frac{15-3x}{3}$
$\therefore y=5-x$
$\therefore$ Both the tables are the same.

$x$	$0$	$5$
$y$	$5$	$0$

$\therefore$ Plot the ordered pairs $(0, 5)$ and $(5, 0)$ of the solution set of $x+y=5$ (or $3x+3y=15$,i.e.,$x+y=5$) on the graph paper and draw the line by joining them.
Here,the graphs of both the equations are the same. Also,we can see that there are infinite points on the line,and they all make the solution set. Thus,the solution set of the pair of linear equations is ${(x, y) \mid x+y=5, x, y \in R}$.