A whole number is any number you can make by adding any number of 1s to 0 -- including 0 itself. Some examples of whole numbers include 2, 5, 17, and 12,000. Rounding is a process by which you take a precise number and re-state it as an approximation. One common means of rounding is using a number line, a visual representation of where a decimal falls in comparison to the whole numbers on either side. However, when you round a decimal number to a whole number, you only need to pay attention to the tenths digit of the decimal to determine which direction to round.
Rounding to a Whole Number
Locate the tenths digit of the number you are rounding. The tenths digit of a number is the digit directly to the right of the decimal point.
For example, the tenths digit of 6.178 is 1. The tenths digit of 7.6 is 6.
Determine whether the tenths digit is less than 5. This will determine whether how you round your number up or down. If the tenths digit is 5 or greater, you round up. If the tenths digit is less than 5 -- including 0 -- you round down.
When the tenths digit is 5 or greater, you round up. This means you add 1 to the ones digit of your number, the digit directly to the left of the decimal point. You then re-write your number without any digits to the right of the decimal point.
Take 43.78. The ones digit is 3, and the tenths digit is 7. Since 7 is 5 or greater, you add 1 to the ones digit, getting 44.78. You then re-write the number without the digits after the decimal point: 44. So 43.78 rounds to 44.
If the tenths digit is 4 or less, including 0, you round down. The ones digit remains unchanged, and you re-write the number without the digits to the right of the decimal point.
Say you're rounding 102.198. The tenths digit is 1, which is 4 or less, so you will round down. Re-write 102.198 without the decimal places: 102. So 102.198 rounds to 102.
TL;DR (Too Long; Didn't Read)
Note that negative numbers are not whole numbers. However, the same rules apply for rounding to the ones digit of a negative number: add 1 if the tenths digit is 5 or higher, do not change the ones digit if the tenths digit is 4 or lower.