math · 24 minutes · Official OETT Aligned
Decimals & Rounding
Keep the right number of decimal places
What to know
- tenths: first digit after the decimal
- hundredths: second digit after the decimal
- lining up decimal points
- rounding the final answer
The place named in the question tells you which digit to keep.
The core rule
Look one digit to the right of the place you need. If it is 5 or more, round up; if it is below 5, keep the digit.
Line up decimal points when adding or subtracting. Keep the full numbers while working, then round the final answer. For 18.746 rounded to tenths, keep the 7 and check the 4 beside it. Since 4 is below 5, the answer is 18.7.
Worked examples
Example 1: easy
18.746 rounds to 18.7 at the tenth.
- Mark 7 as the tenths digit in 18.746 and check the adjacent hundredths digit 4.
- Because 4 is below 5, keep 7 and write 18.7.
The digit just to the right tells you whether to round up.
Example 2: medium
18.746 rounds to 18.75 at the hundredth.
- Mark 4 as the hundredths digit in 18.746 and check the thousandths digit 6.
- Because 6 is at least 5, raise 4 to 5 and report 18.75.
Check one place beyond the digit you need to keep.
Example 3: hard
7.895 + 0.466 = 8.361, which rounds to 8.36 at the hundredth.
- Line up decimal points and add: 7.895 + 0.466 = 8.361.
- For hundredths, check the thousandths digit 1; keep 6 and report 8.36.
Add the full numbers first, then round once.
Common mistakes
Round every extra digit one at a time.
Only the digit immediately to the right of the place you need decides the rounding.
Put it into practice
Use guided explanations, check your answers, and try a new variation.