@mukundmadhav,



He has given in the question that in N(CH3)3, there is positive charge on N, means no lone pair. So IT WONT SHOW +R EFFECT. IT WILL SHOW ONLY -I.



And also in the case of NHCOCH3, dont forget that the lone pair on N is also in continuous conjugation with C=O and as O is more elevtronegative than N, it will withdraw the lone pair of N which in turn will withdraw the electron density from the ring. so IT WILL SHOW -R EFFECT.