A water-treatment team adds a radioactive tracer to a channel to follow the movement of water. The tracer is released 2 hours after it is prepared, and monitoring ends 10 hours after release. For the monitoring result to be usable, the activity then must be at least 30 MBq30\ \mathrm{MBq}30 MBq.
An environmental check takes place 30 hours after preparation. The maximum permitted activity at this check is 4.0 MBq4.0\ \mathrm{MBq}4.0 MBq.
The tracer has a half-life of 6 hours. Its activity changes only by radioactive decay.
The activity when the tracer is prepared is called its initial activity.
Calculate the range of initial activities that would meet both requirements.