This is a relatively easy problem, so maybe the geniuses can refrain from answering right away (PM me if you know the answer Ganch, Data, Justin, Donjuan, etc...). For everyone else:
You have 3 bags of coins with an unknown # of coins in each. 2 of the bags are fake (fool's gold), while 1 bag has real gold coins. The coins are identical except that the real gold coins weigh 1.1 oz per coin while the fake coins weigh 1.0 oz each. You have a very accurate scale, but are allowed just one weighing. How do you determine the bag of real gold coins for sure with just the one weighing?
You have 3 bags of coins with an unknown # of coins in each. 2 of the bags are fake (fool's gold), while 1 bag has real gold coins. The coins are identical except that the real gold coins weigh 1.1 oz per coin while the fake coins weigh 1.0 oz each. You have a very accurate scale, but are allowed just one weighing. How do you determine the bag of real gold coins for sure with just the one weighing?