How would I round 0.598958333 to the thousandths?

Would it be 0.6000 or 0.5990?

Also, could you please tell me why the answer is what it is? If I had something like, 0.55998, as in two 9s in the row, what would I do then?

Thank you!

It is 0.600

The 3rd decimal place is the thousandths. You need the two trailing zeros to specify the precision of the number.