N1V1 = N2V2 is always positive but M1V1 may or may not be equal to M2V2,Why?

Answer 1

no idea - what is the relation between N1,N2 and M1,M2?

It's your job to say it so clearly you cannot be misunderstood.
Just a quick check to see whether you're trying to cheat

Answer 2

Yes, n1v1 = n2v2 always but M1V1 = M2V2 only part of the time. n stands for normality while M stands for molarity. The normality of a solution has been corrected for the valence factor in a balanced equation whereas molarity has not. Another way of saying it is that the equivalent weight is not the same as the molecular weight. For example; in the titration of H2SO4 with NaOH the equation is
H2SO4 + 2NaOH ==> Na2SO4 + 2H2O
If the molarity of H2SO4 is 1 M the normality is 2 N; therefore, M1V1 is not equal to M2V2 but N1V1 = N2V2 every time.