it's a pity that i have to find the answer from different site n post it here for everyone's benefit. i do not wat hapenned for goiit. I EARLIER POSTED THE SAME QUESTION, but didn't get the correct answer so i asked  it as a expert question here, but no ans here too...........

so iam answering my own question now..........

2NaHco₃ ------------- >  Naco₃ + co₂+ H₂O

168 gms ------------> 44 co₂   ( for loss in weight, CO₂ gas must be taken into account)

x gms -------------> 66 gms of co₂

x = 252 gms of NaHco₃  by solving

=> 252 x 100/80 (as the mixture contains  80% of NaHco₃ ) = 315 gms....

=>  360 - 315 = 45 gm of  Naco₃

please correct if it wrong.......