Five books seven papers to weather …

Let the cost of 1 book be Rs.x and that of 1 pen be Rs.y.
Then, according to the question,
5x + 7y = 79 ...(1)
and 7x + 5y = 77 ...(2)
Let us draw the graphs of the equation (1) and (2) be finding two solutions.
These two solutions of the equations (1) and (2) are given below in table 1 and table 2 respectively.
For equation (1) 5x + 7y = 79
{tex}\Rightarrow{/tex} 7y = 79 - 5x {tex}\Rightarrow y = \frac{{79 - 5x}}{7}{/tex}
Table 1 of solutions

For equation (2) 7x + 5y = 77
{tex}\Rightarrow{/tex} 5y = 77 - 7x
{tex}\Rightarrow y = \frac{{77 - 7x}}{5}{/tex}
Table 2 of solutions

We plot the points A(6, 7) and B(-8, 13) on a graph paper and join these points to form the line AB representing the equation (1) as shown in the figure. Also, we plot the points C(1, 14) and D(-4, 21) on the same graph paper and join these points to form the line CD representing the equation (2) as shown in the same figure.
 
In the figure, we observe that the two lines intersect at the points A(6, 7). So, x = 6 and y = 7 is the required solution of the pair of linear equation formed, i.e., the cost of 1 book is Rs.6 and of 1 pen is Rs.7.
Therefore the cost of 1 book and 2 pens = 6 + 2 {tex}\times{/tex} 7 = Rs.20.