What is 4080.2679 rounded to the nearest thousandth

4080.268
the 1st # after the period is the tenths place, 2nd is the hundredths, 3rd is the thousandths, and so on
when rounding you go to the # right of the place in which you want to round to. in this instance you would go to the 9 (ten thousandth place) and use it to round up the 7 in the hundredths place to an 8 because 9 > 5 (# < 5: round down).