Consider the Transportation Problem in the Data Set and Optimal

Consider the transportation problem in the data set and optimal solution below.
Calculate improvement indices on each empty cell. Is this solution optimal?
$\begin{array}{|l|r|r|r|r|r|}\hline \text { COSTS } & \text { Dest. 1 } & \text { Dest. 2 } & \text { Dest. 3 } & \text { Dest 4 } & \text { Supply } \\\hline \text { Source 1 } & 12 & 18 & 9 & 11 & 100 \\\hline\text { Source 2 } & 21 & 7 & 30 & 15 & 150 \\\hline \text { Source 3 } & 8 & 10 & 14 & 16 & 50 \\\hline\text { Demand } & 80 & 60 & 70 & 90 & 300/300 \\\hline & & & & & \\\hline \text { Shipments } & \text { Dest. 1 } & \text { Dest. 2 } & \text { Dest. 3 } & \text { Dest. 4 } & \text {Row Total } \\\text { Source 1 } & 30 & 0 & 70 & 0 & 100 \\\text { Source 2 } & 0 & 60 & 0 & 90 & 150 \\\text { Source 3 } & 50 & 0 & 0 & 0 & 50 \\\text { Col. Total } & 80 & 60 & 70 & 90 & 300/300 \\\hline\end{array}$