Anabelle is a clever trader of rare artefacts. Each day she carries three boxes each filled with thirty artifacts. The boxes can't hold more than that. She travels far in northern lands to sell these artefacts but on the way, she comes across thirty checkpoints where she has to shed one of the artefacts for each sack to the authorities for letting her pass.
How many artifacts will be left with her when she reaches her destination crossing all the checkpoints?
