Solution example for 3 by 4 problem:

Initial feasible solution.

INITIAL A B C D SUPPLY
Source1 21 - 22 11 54
Source2 - 32 - 35 67
Source3 - - 29 - 29
DEMAND 21 32 51 46

Final (optimal) solution:

FINAL A B C D SUPPLY
Source1 21 11 22 - 54
Source2 - 21 - 46 67
Source3 - - 29 - 29
DEMAND 21 32 51 46

Total Cost is 4479