Puzzle #126: Which weighing machine is faulty?

#126 More chocs-a-weigh

Quality Control at the WonderSnack chocolate factory used to have a problem with one of the six machines that produce 200 gram chocolate bars (see puzzle #109 Chocs-a-weigh, 17 April, p 52).

That faulty machine has been scrapped, but Hazel Nutt, the new quality control manager, has discovered that the same issue has cropped up in at least one of the remaining five machines. Faulty machines are producing bars that are each 5 g below the required 200 g weight.

“Hmm,” thinks Hazel. “Last time, we worked out which machine was at fault with a single weighing of a selection of bars taken from each machine. I think…