2005/12 = 167 remainder 1.
So, if 167 of the larger boxes are used 1 bar will remain.
It follows that if 166 larger boxes are used there will13 bars left
(not a mulitple of 5).
However, if 165 of the larger boxes are used this will leave 25
bars, which can be packed into 5 of the smaller boxes.
Total number of boxes is 165 + 5 = 170.
This problem is taken from the UKMT Mathematical Challenges.