where is "that"?

[littlebaby]

hi, can anyone tell me which of the following(s) is correct?

1. I don't know that fifteen thousands mean 15000.
2. I don't know that fifteen thousands means 15000.
3. I don't know fifteen thousands that mean 15000.
4. I don't know fifteen thousands that means 15000.
5. I don't know fifteen thousands mean that 15000.
6. I don't know fifteen thousands means that 15000.

[Tdol]

I don't know that fifteen thousand means 15,000. (Though it's a slightly odd sentence)

[Casiopea]

Additionally, given the fact that you now know the following information, that fifteen thousand and 15,000 are one and the same, use past tense "didn't",

I didn't know (the following information:) that X means Y, but now I do.