Simultaneous equations are two linear equations that can be solved using bar models. By remembering that the same letter will always have the same value we can work out the missing values.
A pair of equations like this are called simultaneous equations - because you are trying to solve them both with the same values for \(x\) and \(y\).
For more information on these methods, see the references at the end of this chapter. There are two fundamental methods of estimation for simultaneous equations: least squares and maximum likelihood.