A hundredth is 0.01
A hundred is 100.
You look at the digital to the right of the 1, in the example above. If that digit is 5 or greater, round up, other round down.
In both cases, you will look at the 0, therefore you would round down.
1800 to the nearest hundredth is 1800.00.
1800 to the nearest hundred is 1800.