What happens, is that 9 rounds up to 10, but if you did that in any position to the right of the first digit in the number this is what happens:

Let's take your number 9,999.999:

If we round the last digits up to the nearest one, we see that we have the following chain reaction:

No matter where you start to round up, you will always end up with

I hope this makes sense.

For example, if you measure someone's height. We usually say that someone is 6'1'', 5'7'', 5'10'', etc. Anything we measure is continuous; we can never give an exact value for it. There will always be rounding errors. Also, it wouldn't make sense to say that you're 5'11.1267844568395678923532479234...''(2)Describe two real-world situations in which it makes sense to round numbers.

Another example would be when dealing with money. The number representation of money never exceeds more than 2 decimal points to the right.

For example, we may see that a bottle of soda may be , but not

Hopefully this makes sense!

--Chris