You can round whole numbers to any place. Rounding decimals works the same way: find the two numbers you are between, and pick the closer one.
Idea clave
To round to a place, look at the digit just to its right. If it is 5 or more, round up (add 1 to the place you keep). If it is less than 5, keep the digit. Then drop the digits after the place you kept.
Por qué funciona
$3.46$ is between $3.4$ and $3.5$. Halfway between them is $3.45$. Since $3.46$ is past halfway, it is closer to $3.5$. The digit after the tenths place tells you which side of halfway you are on: 5 or more is the upper half.
Worked examples
Ejemplo resuelto: To the nearest tenth
Round $7.382$ to the nearest tenth.
- The tenths digit is 3. The digit to its right is 8.
- 8 is 5 or more, so round up: 3 tenths become 4 tenths.
- Drop the digits after the tenths place.
Respuesta: $7.4$
Ejemplo resuelto: To the nearest hundredth
Round $12.964$ to the nearest hundredth.
- The hundredths digit is 6. The digit to its right is 4.
- 4 is less than 5, so keep the 6.
Respuesta: $12.96$
Ejemplo resuelto: A 9 that rolls over
Round $4.97$ to the nearest tenth.
- The tenths digit is 9. The next digit, 7, says round up.
- 9 tenths + 1 tenth = 10 tenths = 1 whole. So the ones go from 4 to 5 and the tenths digit is 0.
Respuesta: $5.0$ (which equals $5$)
Error común
Rounding twice: $3.449 \to 3.45 \to 3.5$. Round only once, from the original number: $3.449$ to the nearest tenth looks at the hundredths digit, 4, so the answer is $3.4$. Check on a number line: $3.449$ is below the halfway mark $3.45$.