step 1: we name all the bag of gold coins as #1, #2, #3......#8, #9, and #10

step 2: we put 1 coin from bag #1, 2 coins from bag #2, 3 coins from bag #3.........8 coins from bag #8, 9 coins from bag #9, and 10 coins from bag 10 onto the scale. Find out the total weight.

step 3: the total weight should have been 10 grams X

(1+2+3+4+5+6+7+8+9+10=55) = 550 grams if all coins are the same (10grams each).

step 4: Subtract the total of step 2 from total of step 3.

Conclusion: If step 4 results 1 gram, then bag #1 is the low quality coins, if step 4 results 2 grams, then bag #2 is the one, if step 4 results 3 grams, then bag #3 is the one.......etc.