3. The table below shows the cost of transporting one block of staging from each of two supply points, X and Y , to each of four concert venues, \(\mathrm { A } , \mathrm { B } , \mathrm { C }\) and D . It also shows the number of blocks held at each supply point and the number of blocks required at each concert venue. A minimal cost solution is required.

1. Use the north-west corner method to obtain a possible solution.
   (1)
2. Taking the most negative improvement index to indicate the entering square, use the stepping stone method twice to obtain an improved solution. You must make your method clear by stating your shadow costs, improvement indices, routes, entering cells and exiting cells.
3. Is your current solution optimal? Give a reason for your answer.
   (1)