Solving a System of Equations in Linear Algebra

[kasse]

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In a city there are four one-way streets that cross each other like this:

http://www.badongo.com/pic/1751680

The number of cars that pass every hour is shown.

Show that x=(x1,x2,x3,x4) satisfies a system on the form

Ax=b
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I really have no clue what to to here.

 

[EnumaElish]

There are cars coming in, and coming out of each direction. The difference (out - in) is the number of cars that take a turn into another segment. Similar to a Markov problem.