Water flows from top to bottom.
First there are two pipes, one with $10$ LPM max flow rate, and another with $20$ LPM max flow rate.
They both go to another pipe, which is $20$ LPM max flow rate.
The total flow rate will be $20$ LPM, because of the lower pipe bottleneck.
What would be the flow rate in the two upper pipes?
Picture of this problem:
I thought it may be $10$ LPM each or $13.333333$ in the fatter, $6.6666667$ in the slimmer. Both make sense to me.
I would also like to know which field does this question belong to? I mean more specific than just fluid mechanics. What theoretical material should I read? Any specific topics / equations?