In Hint 3, there is an equation relating activity A, decay constant λ and number of radioactive nuclei N.
A = λN
We are given in one decay, 2.5×10−12 J of energy is produced, how many decays are required to produce 200 J in 1 second?
This number is the activity A.
The number of radioactive nuclei needed can be found using the above equation.
Number of radioactive nuclei is
number of moles × Avogadro Constant
where number of moles is the ratio of mass to molar mass.
I leave you to work out the detail.
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