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
