Setting up a combustion experiment

An engineer working on a combustion experiment needs to create a rectangular plate with evenly spaced holes for the gas to go through.  The plate needs to be 100 cm by 50 cm (to fit into the machine), and the holes must be staggered, 3 cm apart.  One way she thinks of doing is to have the holes drilled in one row, all 3 cm apart.  For the next row, she was thinking of shifting the holes over, so that the holes make the vertices of equilateral triangles, with the previous row.  Then this would continue for the next rows, until the plate is completely drilled with holes.  The object of this problem is to make a diagram with exact measurements of this plate, where the holes should be drilled.