The digit in the thousandths place is 9 and the
first digit to the right is 5. The rules state that if the
first digit to the right is 5 or more add 1 to the digit
in the place rounding off to. So drop the 5 and add
  1 to the 9 in the thousandths place.

        45.999
      +        1
        46.000  return