When a radioactive nucleus emits $1 \alpha$-particle, its mass number decreases by $4$ and the atomic number decreases by $2$. During positron emission, the mass number remains unchanged but the atomic number decreases by $1$. Similarly, during electron emission, the mass number remains unchanged but the atomic number decreases by $1$.

So, the value of $P=92-2-2+1-1-1$

$\Rightarrow P=92-5$

$\Rightarrow P=87$.