Let, $${x_1} \oplus {x_2} \oplus {x_3} \oplus {x_4} = 0$$ where $${x_1},\,{x_2},\,{x_3},\,{x_4}$$ are Boolean Variables, and $$ \oplus $$ is the $$XOR$$ operator.

Which one of the following must always be TRUE?

Consider the following expressions:
$$\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$(i)$$ $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ false
$$\,\,\,\,\,\,\,\,\,\,\,\,$$ $$(ii)$$ $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$Q$$
$$\,\,\,\,\,\,\,\,\,\,\,$$ $$(iii)$$ $$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,$$ true
$$\,\,\,\,\,\,\,\,\,\,\,\,$$ $$(iv)$$ $$\,\,\,\,\,\,\,\,\,\,\,$$ $$P∨Q$$
$$\,\,\,\,\,\,\,\,\,\,\,\,\,$$ $$(v)$$ $$\,\,\,\,\,\,\,\,\,\,\,\,$$ $$\neg QVP$$

The number of expressions given above that are logically implied by $$P \wedge \left( {P \Rightarrow Q} \right)$$) is _____________.

Consider a set $$U$$ of $$23$$ different compounds in a Chemistry lab. There is a subset $$S$$ of $$U$$ of $$9$$ compounds, each of which reacts with exactly $$3$$ compounds of $$U.$$ Consider the following statements:

$$\,\,\,\,\,\,\,{\rm I}.\,\,\,\,\,$$ Each compound in $$U \ S$$ reacts with an odd number of compounds.
$$\,\,\,\,\,{\rm I}{\rm I}.\,\,\,\,\,$$ At least one compound in $$U \ S$$ reacts with an odd number of compounds.
$$\,\,\,{\rm I}{\rm I}{\rm I}.\,\,\,\,\,$$ Each compound in $$U \ S$$ reacts with an even number of compounds.

Which one of the above statements is ALWAYS TRUE?

Which one of the following well-formed formulae in predicate calculus is NOT valid?