The difference in elevation between two benchmarks was measured by each of three field parties using different kinds of leveling instruments. The results are Party 1, DE= 12.46 +/- 0.04ft, Party 2, DE= 12.43 +/- 0.03 ft, Party 3, DE= 12.50+/-0.06 ft. What is the weighted differance in elevation between the two benchmarks ( nearest 0.001 ft)?