Question 1095293
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S - M = 11,
S + M = 73.
------------------Add the two equations.  You will get


2S = 11 + 73 = 84,   which implies  S = {{{84/2}}} = 42 power plants in the South.


Then in Midwest  there are M = 73 - S = 73 - 42 = 31 power plants.
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