What is the difference between ions and free electrons?

They're pretty different. Electrons are in shells around the nucleus of the atom (2,8,8,2). Ions are electrically charged particles formed when atoms lose or gain electrons, hence the term 'ionic bonding' of which occurs between to atoms where one atom will gain or lose electrons to complete it's highest (outer) shell of electrons.
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Free electrons are those which revolve around nucleus in an atom. while bonding this atom gain or donate  electrons and becomes an ion.it is this transfer of electrons resulting in an ion formation

Adozen is 1221/12 = 1.751.75 x 12 = 21therefore, 1.75 is the answer.