I am trying to understand how do I get approximation for the activity for a radioactive nuclide that produced during a chain while the activity of the source is given at time $t_0$. For example lets take Sr90 wich decay to yittirium90 through $\beta ^-$ decay and the yittirium90 decay to zirconium-90 through a $\beta ^-$ decay.
Lets say that the activity of sr90 at its time production $t_0$ is given as $B(t_0)[Bq]$ Now, if I want to find the actvity of sr90 at time $t$ is just equal to $$B(t)= \frac{N(t)}{N_A}\cdot M $$ while M is the molar mass of sr90, $N_A $ is avogadro's number and $N(t)$ is the number of sr90 atoms in the material at time $t$, which given by $N(t)=N_0 \cdot e^{-\lambda t}$, $\quad\lambda $ is the probability per unit time of a single sr90 atoms to decay and can be taken from internet.
The above is clear for me, what I would want to know now is how do I get an estimate for the activity of yittirium90 at time $t$.