(a) You can find the weight of 3 bags with a single measure. You put 100 coins of the first, 10 coins of the second and 1 coin of the third to the digital scale. Its at most 999 grams. The weight of the coins from the first bag will be the the first digit of the result, the weight of the coins from the second bag will be the second digit and the weight of the coin from the third bag will be the third digit. (b) Each weight of the coins can be from 0 to 9 therefore there are 10^30 starting positions. After each use of the scale, the result is between 0 and 999. 9 measurement means 1000^9 possibility which is less than 10^30 so you will get the same result in different starting positions and you cannot decide which one you have got.