1. I stayed there for two weeks.

Question

2. I stayed there two weeks.

Are both grammatical?

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3. This is the way how he solved the problem.

4. This is the way that he solved the problem.

5. This is the way he solved the problem.

6. This is the way in which he solved the problem.

#3 is wrong, but #4 is right. Is that true? Then how come #3 is not grammatical? #4 and #5 are grammatical, aren't they?

Answer 1

Yes, among these six sentences, only #3 is incorrect.

You can make #3 correct in either of these ways:
This is how he solved the problem.
This is the way he solved the problem.