Итак, это просто, смотри:
a: Whose camera is this?
b: It's Bob's. It's his camera. It's his.
a: Whose watch is this?
b: It's Tony's. It's her watch. It's hers.
a: Whose skateboard is this?
b: It's Bill's. It's his skateboard. It's his.
a: Whose football is this?
b: That's Paul's and Anna's. That's their. That theirs.
a: Whose alarm clock is this?
b: It's Pat's. It's his alarm clock. It's his