You should number each bag as 1,2,3,4,5,6,7,8,9,10 respectively.
you must remove the number of coins which are written on that bag.
if a bag has the no. 5 written on it remove 5 coins from it.
so if there are no counterfeit coins are there weight should be:
10 x (1+2+3+4+5...+10)=10 x 55=550
then we can find the answer by trial and error.
if 1 numbered bag has counterfeit coins then the weight will be 549.
if 6 numbered bag has counterfeit coins then the weight will be 544 and so on.
hope it helps and pls mark as best answer.