Anabelle is a clever trader of rare artifacts. Each day she carries three boxes with each filled with thirty artifacts. The boxes cant hold more than that. She travels far of northern lands to sell these artifacts but on way, she comes across thirty checkpoints where she has to shed one of the artifact 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 check points ?

Anabelle is no dumb trader and she knows the strategy with the help of which she will be able to save 25 artifacts even after passing 30 check points.

All she will focus on is getting rid of the boxes as soon as possible.

To shed the first sack quickest possible, she will start filling artifacts from one box to other two. Assume that she is able to do that after passing through A check points.

(Space in first box) A + (Space in second box) A = (Remaining artifacts in third box) 30 -A

A = 10

Therefore, after 10 check points, she will be left with two boxes with 30 artifacts each. Now to get rid of the second box, she will start filling artifacts from second box to first box. Let us assume that she is able to get rid of the second box after B checkpoints.

(Space in first box) B = (Remaining artifacts in second box) 30 - B

B = 15

Therefore, after fifteen check points, she will be left with one sack with thirty artifacts in it.

Since she has five more check points left, she will have to give five more artifacts and after passing them all, she will still have 25 artifacts.