I sometimes visit Puzzling StackExchange, and this time I found this puzzle. Which I found particuarly interesting, because I suspect that maybe it can be solved with linear equation systems.
To formulate the question:
Assume we have a set of hourglasses $\mathcal H = \{h_1,h_2,\cdots,h_n\}$ running for $\{m_1,m_2,\cdots,m_n\}$ minutes.
How can we arrange their flipping to measure $N$ minutes?