please help

4. Six tubes are placed in a rack. To all six tubes, you add 2ml of water. The 1ml of serum is added to the first tube and mixed well. You then transfer 1ml of that to the second tube and again mix well. You continue the transfer of 1ml of mixture from one tube to each subsequent tube, finally discarding 1ml from tube 6 . What is the dilution of serum contained in tube 6