Question

the total cost of 5 book and 7 pens is 79 rupees and the total cost of 7 book's and 5 pens is 77 rupees.find the cost of 1 book and 2 pens​

Answer 1

Let the cost of one book = x

Let the cost of one pen = y

According to question :

1st Condition :

5x + 7y = 79....(1)

2nd Condition :

7x + 5y = 77...(2)

Add Equation (1) and (2)

Then

12x + 12y = 156

x + y = 13...(3)

Subtract (1) from (2)

Then

2x - 2y = 2

x - y = 1...(4)

From Equation (4)

x - y = 1

x = y + 1...(5)

put the value of x in equation (3)

x + y = 13

y + 1 + y = 13

2y + 1 = 13

2y = 13 - 1

2y = 12

y = 12 /2

y = 6

put the value of y in equation (5)

x = y + 1

x = 6 + 1

x = 7

Cost of one book = x = Rs. 7

Cost of two pen = 2y = 2(6) = Rs. 12

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