The diagram below is a map of a system of one-way streets. Sensors have been
installed at four points to monitor the volume of trac entering and leaving this
system. The numbers given are the number of vehicles per hour recorded at those
points. The variables x1, x2, x3 and x4 represent the number of vehicles per hour
travelling along the remaining streets.
Write down a system of linear equations linking x1, x2, x3 and x4.