Radioactive decay is random, but a large sample is very predictable. In one half-life, half of the undecayed nuclei decay, so the activity drops to half. Wait another half-life and it halves again. Wait the right number of half-lives to bring the activity down to the target.
MissionWait until the activity falls to 200 counts/s.Round 1Streak 0Best 0
Compared with the target, have you waited:
The sample is decaying away. Predict, set the number of half-lives, then check.
half-life of this source8 days
time waited0 days
fraction remaining = (1/2) to the power n1
activity now800 counts/s
Each half-life, the activity falls to half: 800, 400, 200, 100, ...