Mathematics · Ratios and Proportions

A protein solution (0.2 ml) of unknown concentration was diluted with 0.8 ml of water to make a total volume of 1.0 ml. To 0.5 ml of this diluted solution, 4.5 ml of biuret reagent was added and the color developed. The absorbance of this mixture in a 1 cm cuvette at 540 nm was observed to be 0.20. Separately, 0.5 ml of a standard BSA (4 mg/ml) solution plus 4.5 ml of biuret reagent gave an absorbance of 0.20 under the same conditions. What is the protein concentration (in mg/ml) in the original undiluted unknown solution?