Question

5 books and 7 pens together cost rs.79 where as 7 books and 8 pens together cost rs.77.Find the cost of 1 book and 2 pens.​

Answer 1

Given :- Cost Of 5 Books and 7pens = Rs . 79

Cost of 7 Books and 8 pens = Rs . 77.

To Find :- Cost Of 1 Book + 2 Pens .

Solution :- Let , Cost of 1 Book = Rs. x

Cost of 1 Pen = Rs. y

Case I :- 5x + 7y = 79 -- ( i )

Case II :- 7x + 8y = 77 -- ( ii )

We can Solve It from any method to solve A pair of linear equation . But here I am applying the Elimination Method . But to do Elimination Method we want x or y to be equal in both equations . So , Here I am going to multiply Equation ( i ) by 7 on both sides and Equation ( ii ) by 5 on both sides . After multiply the equations are as follows :-

35x + 49y = 553 -----( iv )

35x + 40y = 385 -----( v )

Subtracting ( v ) from ( iv )

35x + 49y = 553 - ( 35x + 40y = 385 )

The equation that we get after Subtracting these equations will be :-

9y = 168

y = 18.67 rupees

Now ,. By eq. ( i ),

5x + 7y = 79

5x + 7 × 18.67 = 79

5x + 130.69 = 79

5x = 79 - 130.69

5x = -51.69

$x = \frac{ - 51.69}{5}$

x = -10.338 .

Therefore , Your Question is wrong because rupees cant be counted in -ve .