From stock solution of 1000ug/ml of antibiotic I need to prepare 2.500ul of solution of 55ug/ml concentration.

Answer 1

x(1,000 μg/ml) = (2.5 μl)(55 μg/ml)
x(1,000 μg/ml) = (0.0025 ml)(55 μg/ml)
1,000x μg = 0.1375 μg
x = 0.1375 μl with 2.3625 μl pure H20