Rewrite the sentences in passive voice. 1)the sister often takes bob for his brother 2)i don't know how we can solve

 1. Bob is often taken for his brother by the sister. 

 2. I don\'t know how this problem can be solved.

 3. Unfortunately the tickets can\'t be found by me.

 4. We are not advised by them to go there so often.

 5. They are often written letters to their friends.