okay the answer, i do not think this was the hardest so far, but i'll give you the answer
you give every wood-cutter a number from 1 to 10. then you take from nr 1, 1 piece of wood, from nr 2 you take 2 pieces etc. until you take 10 from nr 10. at the end you should get 1+2+3+4+5+6+7+8+9+10=55 kg of wood. but the scale will say you miss some gram(s) so if it sais 54,994 kg. then you miss 6 grams, so that will be nr 6, because he gave 6 pieces that way 1 gram too less. and if the scale sais 54,990 kg, you miss 10 grams so it's nr 10 because he gave 10 pieces that way 1 gram too less