posted by alex .
A 150.0-mL sample of Solution A is diluted to a total volume of 423 mL to form Solution B. If the concentration of Solution A was originally 20.0 mg/L, what is the concentration (in mg/L) of Solution B?
20.0 mg/L A x (150.0/423) = concn B in mg/L.
Or you can use the dilution formula which is
mL1 x M1 = mL2 x M2.