3. The table below shows the cost, in pounds, of transporting one tonne of concrete from each of three supply depots, \(\mathrm { A } , \mathrm { B }\) and C , to each of four building sites, \(\mathrm { D } , \mathrm { E } , \mathrm { F }\) and G . It also shows the number of tonnes that can be supplied from each depot and the number of tonnes required at each building site. A minimum cost solution is required. The north-west corner method gives the following possible solution. Taking AG as the first entering cell,

1. 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.
2. Determine whether your current solution is optimal. Justify your answer.