Why one of them is wrong whereas the other one is correct?

Hello everyone.

I have a question for what I couldn't get any enough-explanatory answer.

There is a usage like "adjective+noun+to+verb".

Can we use them in these kinds of sentences?

1- We are talking about a hard problem to solve.

2- We saw a nice car to travel in.

3- Would you like to read a difficult book to understand?

and so on. We can reproduce them like that.

Do you think that those sentences are correct and have the same meaning as the following ones?

4- We are talking about a problem which is hard to solve.

5- We saw a car which is nice to travel in.

6- Would you like to read a book which is difficult to understand?

Thank you very much.