2.51

1) Find an empty bag (labeled "E")
2) Place 1 coin from bag 1 into E
3) Place 2 coins from bag 2 into E
...
10) Place 9 coins from bag 9 into E
11) Place 10 coins from bag 10 into E
12) Weigh bag E on your digital scale

If all coins were 10 grams, the bag would weigh 550 grams. Thus, [math]\displaystyle{ 550 - weight }[/math] will tell you how many coins are too light. Since this number of coins correlates to the bag from which the coins came, you now know which bag contains the light coins.

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