It reminds me when elecrical resistors are in parallel: there you have to use reciprocals (for the same reasons) too.
Precisely. As the voltage across parallel resistors is identical, we are considering the total flow of current through the various resistors, just like multiple taps dripping into a basin, or workers hopefully working alongside each other.
How does any of this help you to fly a plane ?
Exactly the point I made at the end of my initial solution - its of no use to aviation whatsoever! Just a stupid test of irrelevant maths, where there are far more useful aspects, eg mental arithmetic or logic, which could have been tested.
But inner simply wanted help with seeing how to solve the problem, so I obliged.
PM