Question

A shopkeeper increases the list price of a book by 10.rs . A person buy 10 books less for 1200.rs . Now find the original value of list price of the book? ​

Answer 1

For this one has to know the basics of quadratic equations.

The answer is as follows:

Let the number of books that can be bought in 1200 rs be x. And let per book cost be y rs. Then

xy=1200………..eq 1

Now we have that if per book cost is increased by 10 rs i.e. y+10 rs, then the number of books that can be bought in 1200 rs reduce by 10 i.e. x-10 books.

Then

(x-10)(y+10)=1200

Or, xy+10(x-y)-100=1200

now we know that xy=1200

Therefore,

1200+10(x-y)-100=1200

Or, x-y=10

Or, x=y+10

Putting the value in eq 1 we get

y(y+10)=1200

Or, y^2 +10y-1200=0

Or, y^2+40y-30y-1200=0

Or, y(y+40)-30(y+40)=0

Or, (y-30)(y+40)=0

Therefore, y-30=0 or y+40=0

Or, y=30 or y=-40

Since y cannot be nrgative thus we have

y=30 rs

Therefore, the original list price of book =y=30 rs