"p" notebooks costed N200  ====>  hence, one notebook price was  before increasing.


After increasing, one notebook price becomes , and the difference is N10.


Equation


     -  = 10


To solve it, multiply both sides by  p*(p-1). You will get


    200*p - 200*(p-1) = 10p*(p-1)


    200p - 200p + 200 = 10p^2 - 10p


    10p^2 - 10p - 200 = 0


    p^2 - p - 20 = 0


    (p-5)*(p+4) = 0.


The only positive solution is  p= 5.


Answer.  Before price increasing, one notebook cost was   = N40.