[Grammar] There are 7 days in a week.

[Yoshio]

Would someone check the below sentences, and tell me whether or not they are grammatically OK?

There are 7 days in a week.
A week has 7 days.
7 days is a week.

A week is 7 days.
7 days are a week.

Thank you in advance,

Yoshio

 

[5jj]

The first two are fine.

 

[emsr2d2]

Additionally, I would accept "A week is seven days" although it is not as natural as the first two.

 

[Tdol]

7 days is a week.

You could use make there.

 

[dut_thinker]

Seven days constitute a week.

 

[Tdol]

Seven days constitute a week.

It doesn't sound that natural to me to use this verb- it sounds a bit too formal.

 

[Yoshio]

Thank you all for useful information.
So the following two are not right.

Seven days is a week.
Seven days are a week.

Thank you again for your help. I really appreciate it.