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I was wondering if anyone could help me with this question ...

"A device used in radiation therapy for cancer contains 0.50g of cobalt (60-Co-27). The half life of cobolt is 5.27 years. Determine the activity of the radioactive material."

Working out:

(To find the decay constant)

T1/2= ln2/[lamba]

[lamba]= ln2/5.27 years

=0.1315

(To find activity)

dN/dt= -[lamba]N

...

How do you calculate for N?! (I've been trying to use the Avogadro's number but keep on obtaining really bizarre numbers ... Perhaps I'm not using it correctly)

N=0.0005kg / 60u (60 x 1.6605x10^27) x Avagadro's Number

The correct answer is: 2.1x10^13 Bq

Any help will be much appreciated