Lesson 4: Round Decimals

Property

To round a decimal to a given place value, look at the digit to its immediate right. If the digit is 5 or greater, round up the digit in the target place. If the digit is 4 or less, keep the digit in the target place the same. All digits to the right of the rounded digit are dropped.

Examples

• Rounding 1.58 doubloons to the nearest tenth: The digit in the tenths place is 5. The digit to its right is 8. Since $8 \geq 5$, we round the tenths digit up to 6. So, 1.58 rounds to 1.6.
• Rounding 23.741 doubloons to the nearest hundredth: The digit in the hundredths place is 4. The digit to its right is 1. Since $1 < 5$, we keep the hundredths digit the same. So, 23.741 rounds to 23.74.

Explanation

Rounding is a way to estimate a number by making it simpler while keeping its value close to what it was. The process involves deciding whether to increase a digit or keep it the same based on the value of the digit to its right. This is useful for dealing with money, measurements, or any situation where exact values are not necessary.

Property

To round a quantity in a word problem, first identify the number to be rounded and the target place value. Then, apply the standard rounding rules: look at the digit to the right of the target place value. If it is 5 or greater, round up; if it is less than 5, round down.

Examples

• A pirate captain has $178.26$ doubloons. Rounded to the nearest whole doubloon, the captain has $178$ doubloons.
• A treasure map lists a buried chest at $34.7$ paces north. To the nearest ten paces, the distance is $30$ paces.
• A ship's cook uses $2.548$ kilograms of flour. Rounded to the nearest hundredth of a kilogram, this is $2.55$ kg.

Explanation

Rounding is often used in real-world situations to make numbers simpler and easier to work with. When a problem asks for an approximate value, "about," or "to the nearest," it is a signal to round. You must identify which number to round and to what specific place value (like the nearest ten, whole number, or tenth). Applying rounding rules in context helps in estimating quantities, measurements, and money.