1a. She is wearing a watch.
1b. She is wearing a watch on her wrist.
Note, a watch is usually worn around one's wrist, or on one's wrist.
2a. The one reading a book is Helen.
2b. The one who is reading a book is Helen.
3b. seventy five (people use it, but the hyphen is best. It helps to show that the two words are related0
4. ...when he found (that) his assistant had taken ...