Let the cost of a book be Rs x and that of a pen be Rs y. Then,
5x + 7y = 79 .....(i)
7x + 5y = 77 ....(iii)
Multiplying equation (i) by 5 and equation (ii) by 7, we get
25 + 35y = 395 ....(iii)
49x + 35y = 539 .....(iv)
Subtracting equation (iii) by equation (iv), we get
49x - 25x = 539 - 395
24x = 144
x = \(144\over24\) = 6
∴ Cost of a book = Rs 6
Putting x = 6 in equation (i), we get
5 x 6 + 7y = 79
30 + 7y = 79
7y = 79 - 30
7y = 49
y = \(\frac{49}{7}=7\)
∴Cost of a pen = Rs 7
∴ Cost of 2 pens = 2 x 7 = Rs 14
Hence, the total cost of 1 book and 2 pens = 6 + 14 = Rs 20
