There is not an optimal solution to this, as far as I know.
If you consider the board to be invisible squares of 70cm x 70cm, 30 of them side by side, you can then look at packing circles in squares, and you have the squares then 5 diameters x 5 diameters. Now for squares, there are optimal solutions http://hydra.nat.uni-magdeburg.de/packing/csq/d3.html and you can see for each square N=25, and you have 30 squares, so 30x25.
This math of packing is a very challenging field, and of much commercial value. Box sizes are determined with this math.
For area, compute board area, subtract the areas of the circles.
a rectangle of board measures 21 m by 70 cm. Circular plates are to be cut from the board of diameter 14 cm. how many plates and board is left
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