Total number of lone pairs in PCl₅ is a, in ${\mathrm{PCl}}_4^{+}$ is b and in ${\mathrm{PCl}}_6^{-}$ is c. 

Value of $\mathrm{a}+\mathrm{b}+\mathrm{c}$ is ____. 

There is no lone pairs in PCl₅  as all five valence electrons of P are involved in bonding. Value of a is zero.

Positive sign in ${\mathrm{PCl}}_4^{+}$ indicates the presence of 4 electrons instead of five and all electrons are involved in bonding. Hence, no lone pairs in ${\mathrm{PCl}}_4^{+}.$ Value of b is zero.

Negative sign in ${\mathrm{PCl}}_6^{-}$ indicates the presence of 6 electrons instead of five and all electrons are involved in bonding. Hence, no lone pairs in ${\mathrm{PCl}}_6^{-}$. Value of c is zero.

$\mathrm{a}+\mathrm{b}+\mathrm{c}=0+0+0=0$