Find linear programming problem given the final optimal tableau

I have been given this problem and cant find a solution and was wondering if anyone could help! I have looked in to matrices and artificial variables but I am unsure how to approach this problem. Any help would be AWESOMMMME. Many thanks in advance. Mal :)

The __final optimal tableau__ for a maximisation Linear Programming problem with two constraints of type (<=) and two original variables, *x*_{1} and *x*_{2}, is given below. The variables *x*_{3} and *x*_{4} are slack variables.

| **Soln** | *x*_{1} | *x*_{2} | *x*_{3} | *x*_{4} |

*x*_{1} | 20 | 1 | 0 | 2 | -1 |

*x*_{2} | 40 | 0 | 1 | -1 | 1 |

z | | 0 | 0 | 1 | 1 |

Find the associated optimal value of the objective function, and derive the original Linear Programming problem.