If the half-life of 201tl is 3.0 days, what is its activity, in megabecquerels after 3.0 days?

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The correct answer is = 36 MBq

The explanation :

when we have the original activity in megabecquerels = 72MBq

and we have the half-life of 201Tl is 3.0 days.

so the activity after 3.0 days it will be the half of the original value.

so the activity is = 72MBq/2

= 36 MBq