We know the formula N=No(1/2)^t/t1/2

Now there is a question

The activity of a material reduces to 1/64 in 60 sec then t1/2 will be..

when i saw the soln. 

it was written

N/No=1/64

How?

What is t1?Could you be clear?

I think activity=N=Nln2/t_1/2 where N is the no. of nuclei at that time.So,when activity became 1/64th,N becomes 1/64th of initial no.of nuclei since ln2 and t_1/2 don't change.Hence the ans is 360sec.

we have activity = -dN/dt =  N

so it follows easily as  is const .