Answer:
10 days
Step-by-step explanation:
The half-life of a radioactive sample is the time taken for half of the sample to decay.
In the diagram, the half-life corresponds to the time after which the % of cobalt-57 has halved. We can observe the following:
At t=10 days, the % of Co remaining is approximately 45%
At t=20 days, the % of Co remaining is approximately 22%
This means that the sample of cobalt-57 has halved in 10 days, so the half-life of cobalt-57 is 10 days.