Nuclear Physics. Help Me Solve This Question.?

A radiologist finds a radioactive with no date of acquisition on it. He places the sample in front of a canter and records an activity of 16000Bq. He then places a similar sample that has just arrived in front of the canter and records the activity of 128000Bq. How long the unlabelled sample has been in the lab, if the 1/2 life of the material is 6 days?