Question 1148774
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As I understand your post, you just know how to solve a) and b), and your question relates to part c) ONLY.


If so, then simply substitute  y= 17.96 (obtained in a) ) into the given formula and find x from this equation.


So, your equation is in this case


    17.96 = 80.7 - 11*ln(x).


It implies then


    ln(x) = {{{(80.7 - 17.96)/11}}} = 5.70


    x                   = {{{e^5.70}}} = {{{2.72^5.70}}} = 299.9 cubic feet per child = 300 cubic feet per child.


To get understanding why and how this solution works, you simply need to read ATTENTIVELY the condition 
of the problem one, two, three times, until all conceptions become clear to you.
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Solved.