Question 795176
j for juice
s for soda
w for water


Translating from the descriptions,
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{{{j=36}}}.
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{{{s=j-13}}}.
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{{{w=s-10}}}.
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Try to see how those make sense, before continuing.


Do you want to use a matrix to solve, or do you want to use just substitutions and simplifications to solve?  You have a system of three equations in three unknown variables.  


Simplest step first can be substitute for j=36 in the s and the w equations.  
{{{s=36-13}}} and {{{w=s-10}}}
Obviously one of these is affected but not the other one.  Simplify s, and then substitute it into the w equation:
{{{s=23}}}.
{{{w=23-10}}}
{{{highlight(w=13)}}}.  Very Neat!  Solved for w.
Thirteen bottles of water sold.