Question 1112721
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1-st gen produces 10 Mw.


2-nd gen produces 25 Mw.


3-rd gen produces  0.5*(10+25) = 17.5 Mw.


The answer to your question is THIS:   {{{210/(10+25+17.5)}}}.
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From the PHYSICS point of view, the problem formulation is totally/fatally incorrect and is nonsense.


Because it mix two different conceptions: Power and Energy.