Rounding decimals by place value
To round a decimal, first name the digit in the place you are rounding to. Then look only at the digit one place to its right.
A worked example
Round 4.278 to the nearest hundredth. The hundredths digit is 7, and the digit after it is 8, which is 5 or more. So the 7 goes up to 8, and you get 4.28. You drop all the digits after the hundredths place.
The mistake people actually make
4.278 rounded to the nearest tenth is 4.3, not 4.2, because the digit after the tenths place is 7, so you round up. If you name the wrong place, you get the wrong answer even when you follow the rule.