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This can be more understood by taking two test questions and their solution as well.

To find: - We have to find the cost of 12 such books.

Let the cost of 1 book be ‘x’

Cost of 20 books would then be (20) x= 20x … (1)

But according to the question cost of 20 books is 400 ... (2)

Now from the equation (1) and (2), we will get

20x=400

Dividing both sides by 20

20x/20 = 400/20

x= 20, which is the cost of the 1 book

Therefore the cost of 12 books = 12(x) = 12 x 20 =$240.

(10x²-5x +11) – (6x²-12x+15)

To simplify the above equation, first of all we need to open the parentheses and then we need to distribute the negative sign in the 2

=10x²-5x+11-6x²+12x-15

=10x²-6x²-5x+12x+11-15

= 4x²+7x-4.

Therefore 4x²+7x-4 is the solution and the simplified form of (10x²-5x +11) – (6x²-12x+15).