5 books and 7 pens together cost Rs 79 whereas 7 books and 5 pens together cost Rs 77. Find the total cost of 1 book and 2 pens.

Let the cost of 1 book be ‘a’ and cost of 1 pen be ‘b’.

Given, 5 books and 7 pens together cost Rs 79 whereas 7 books and 5 pens together cost Rs 77.

⇒ 5a + 7b = 79 ------ (1) and 7a + 5b = 77

Multiplying eq1 by 7 and eq2 by 5 and subtracting eq2 from eq1.

⇒ 35a + 49b – 35a – 25b = 553 – 385

⇒ 24b = 168

⇒ b = Rs. 7

Substituting value of b in eq1, we get

⇒ 5a + 49 = 79

⇒ a = Rs. 6

Cost of 1 book and 2 pens = 6 + 2 × 7 = Rs. 20