The total cost of two pens is Rs.20/-. If the cost of one pen is Rs.2/- then the cost of other pen is Rs. 18/-. If the cost of one pen is Rs. 4/- then the other pen cost Rs. 16/-

We can write an equation using the variables x and y such that the cost of one pen is x and the other pen is y.

The equation is x+y = 20

x = 2 and y = 18 make the equation true.

Similarly x = 4 and y = 16 also make the equation true.

x = 2 and y = 18 is a solution

x = 4 and y = 16 is an another solution

x = 8 and y = 16 is not a solution

Any Equation of the first degree having two varibles such as x and y is called a linear equation in two variables.