I got this one at the last class at the end of my course in mathematics last semester... I have 10 stacks of gold coins with 10 coins in each of them, but one of these stacks of coins is false and therefore the weight is presumed to differ from that of the non-fake ones. How I am supposed to find out which of these stacks is the false one. To my disposal I have an apparatus with which I can weight the stacks, viz. when I lay down a stack on the weight, the opposite weight of it is rising. Note that I have to do it as fast as it is concievably possible, i.e., weight the stacks as less times as I can. When I say 'weight', I mean that once the certain weight has been weighted, and when that particular weight is either diminished or increased, it is yet another 'weight'.