republic, I think i have the solution, its just in my head i have not tried on paper yet, correct me if I am wrong. soln, you leave one stack, weigh only 9 stacks but with following order 1st stack -- 1 coin 2nd stack -- 2 coins 3rd stack -- 3 coins 4th -- 4 5th -- 5 6th -- 6 7th -- 7 8th -- 8 9th -- 9 if it was all 100 gm coins it would weight (x)=900+800+700+600+500+400+300+200+100 When we weight if the weight is equal to x, then the odd coin is from the stack that we left out. if the weight is 10 gram less its the 1st stack 20 gm less -- 2nd stack 30 gm less -- 3rd stack 40 gm less -- 4th stack 50 gm less -- 5th stack 60 gm less -- 6yh stack 70 gm less -- 7th stack 80 gm less -- 8th stack 90 gm less -- 9th stack I think thats the answer ....... damn it was a good one. thank you republic keep it coming.