Question

and tell why..............

Answer 1

Hey here is your answer
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Let the cost of Notebook be x
and the cost of Pen to be y

then, a/q
the cost of a notebook is twice the price cost of a pen
: x = 2y
: x - 2y = 0
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Answer 2

let cost of notebook be "x"
     cost of pen be "y"
    then the linear equation in two variables is  
                       x=2y