Mathematics hackerank

Can u please help me solving this

Yes this is actually a gcd problem. The amount of water in each jug is always a [linear combination] of a and b adn this linear combination has to be a multiple of gcd of a and b.
See this https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-fall-2010/readings/MIT6_042JF10_chap04.pdf
If you still fail to understand this person solves the exact same problem https://www.youtube.com/watch?v=D-DYtUmRMa4

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