please find the attached photo of the missing part of your question:
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
