SUPPOSE 1 QUESTION -- COST OF PEN = (X-4)(X+4)=0 WHERE X= NO. OF PEN . WE OBTAIN NO. OF PEN IS 4,-4 . BUT PROMBLEM IS THAT NO. OF PEN IS POSITIVE WHY -4 IS FOUND IN THESE ANSWER

But here thing is since you are saying where x are number of pens the relation (x+4)(x-4)=0 does not exist.It is like a vice versa.For you to get -4 you have to have the relation that (x+4)(x-4)=o is true.But here that is not the case.....