Two radioactive nuclei P and Q in a given sample decay into a stable nucleus R. At time $t = 0$, the number of P species is $4N_0$ and that of Q is $N_0$. The half-life of P (for conversion to R) is $1$ minute whereas that of Q is $2$ minutes. Initially there are no nuclei of R present in the sample. When the number of nuclei of P and Q are equal, the number of nuclei of R present in the sample would be: