The third digit in 399 is 9. To round to this place value, we inspect the digit behind it. If the digit is 5 or greater, we round up; if it is less than 5, we round down.
The digit behind 9 is 7. Since it is greater than 5, we round up. This would ordinarily put the number up 1; however, doing that makes the 9 a 10, which in turn makes the next number 10, and the next number 4. This gives us 400,000.
