Answer:
Model A=30, Model B: 20, Model C: 45
Step-by-step explanation:
Let x be the amount of type A mp3 produced, and the amount of model B mp3's produced and z the amount of model C mp3 produced.
Since in total there are 303 hours for electronics work, then:

Since in total there are 393 hours for assembly, then:

Since in total there are 416 hours for quality assurance, then:

Then, the linear system associated to the problem is

with coefficient matrix
and vector of constant terms ![b=\left[\begin{array}{ccc}303\\393\\416\end{array}\right]](https://tex.z-dn.net/?f=b%3D%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D303%5C%5C393%5C%5C416%5Cend%7Barray%7D%5Cright%5D)
Since the determinant of A is equal to 36.704 then A is invertible.
Then for solve the system
, is enough find the inverse of A and operate

Using Octave we obtain that
![A^{-1}=\left[\begin{array}{ccc}-0.22&-0.50 &0.71\\0.33&0.15&-0.34\\-0.028& 0.35&-0.20\end{array}\right]](https://tex.z-dn.net/?f=A%5E%7B-1%7D%3D%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D-0.22%26-0.50%20%260.71%5C%5C0.33%260.15%26-0.34%5C%5C-0.028%26%200.35%26-0.20%5Cend%7Barray%7D%5Cright%5D)
Then
![x=A^{-1}b=\left[\begin{array}{ccc}30\\20\\45\end{array}\right]](https://tex.z-dn.net/?f=x%3DA%5E%7B-1%7Db%3D%5Cleft%5B%5Cbegin%7Barray%7D%7Bccc%7D30%5C%5C20%5C%5C45%5Cend%7Barray%7D%5Cright%5D)
This means that 30 mp3's of model A, 20 of model B and 45 of model C must be produced