Approximately days have passed when 20% of cobalt (co) remains?

The half-life of Cobalt 60 is 5.27 years. This means that after this time, only 50 % of the initial amount of Cobalt is left. The relationship between the number of nuclei left at time t and the time, for a radioactive decay, is given by

where  is the number of nuclei left at time t,  is the number of nuclei at t=0, and  is the half-life. For cobalt, .

We can re-arrange the formula as

and then

the problem asks for the time t at which only 20% of cobalt is left, that means the time t at which

therefore, using this into the previous equation, we get

so, after a time of 12.24 years, only 20% of cobalt is left.