The thousands digit is 8 so we know it is 8,000 The hundreds digit is 2 less than 8, so it's 6, giving us 8,600. The units digit is half the hundreds digit, so it is half of 6, or 3, now we have 8,603. Lastly, the tens digit is one greater than the units digit, so it is 4, leaving you with 8,643.
