Calculate the number of neutrons for the following elements:
`""_18Ar^40`

Calculate the number of neutrons for the following elements: ""_15P^31

Calculate the number of neutrons for the following elements: ""_11Na^23

Calculate the number of neutrons for the following elements: ""_17Cl^35

Calculate the number of neutrons present in the nucleus of an element X which is represented as ""_(15)^(31)X .