The thousandth place is 0.0<u>0</u>. To round, look at the number right after it. If the number is 4 or less, the number stays the same. If the number is 5 or greater, the number is rounded up.
For 66.3899872389, the bold number is the number we are trying to round. The number after it is 9. This number is greater than 5 so you have to round up.
