Answer:
2.5 half-lives
Step-by-step explanation:
Find
in
given 
is the amount after the time
,
`is the initial amount,
is the half-life
We know that after half-life there will be twice less the initial quantity:
.
Simplifying gives 
Plugging this into the initial equation, we obtain that
or 
Finally, just plug in the given values and find the unknown one.
From
, we have that
= 
So you should get: 
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