Chapter Outline

Understanding Decimals

A decimal is a kind of fraction. It expresses a part of a whole.

• The decimal point separates whole numbers from decimal places.
• All decimal place names end in –ths.

To read a whole number and decimal, follow these steps:

• Say the whole number first.
• Say the word and for the decimal point.
• Read the decimal as though it were a whole number.
• Give the decimal a name according to the number of places it holds (tenths, hundredths, thousandths, and so on).

To write a decimal, follow these steps:

• Think about the number of decimal places you need.
• Use zeros to hold places where necessary.
• Remember the word and separates whole numbers from decimal fractions.

Rounding is a useful tool for estimating answers to decimal problems. To round a decimal, follow these steps:

• Underline the digit in the place to which you are rounding. If the digit to the right of the underlined digit is greater than or equal to 5, add 1 to the underlined digit. If the digit to the right of the underlined digit is less than 5, leave the underlined digit as it is. Drop the digits to the right of the underlined digit.

Adding and Subtracting Decimals

To add decimals, follow these steps:

• Line up the decimals, with decimal point under decimal point.
• Add each column.
• Place the decimal point in the answer directly below the decimal points above.

To subtract decimals, follow these steps:

• Line up the decimals, with decimal point under decimal point.
• Subtract.
• Place the decimal point in the answer directly below the decimal points above.
• Use zeros to give each number the same number of decimal places.

Multiplying and Dividing Decimals

To multiply decimals, follow these steps:

• Put the number with more digits on top and the other below.
• Multiply as you would multiply whole numbers.
• Count the number of decimal places in both of the numbers you multiplied.
• Put the total number of decimal places in the answer.

To divide decimals, follow these steps:

• Make the divisor a whole number by moving the decimal point to the right.
• Move the decimal point in the dividend the same number of places.
• Divide as you would with whole numbers.
• Place the decimal point in the answer directly above its new position in the dividend.

To solve decimal problems with a Casio ƒx-260 calculator, press the decimal point key for the decimal point. Do not confuse the decimal point key with the raised dot that means "multiplication."

Scientific Notation with Large Numbers

Scientific notation is a method of writing numbers as a power of ten. To write a large number in scientific notation, follow these steps:

• Move the decimal point in the number so the new number is between 1 and 10.
• Next to the new number, write × 10.
• Count the number of places the decimal point moved.
• Write the number of places that the decimal point moved as a power next to 10.

Decimal Word Problems

To mark a decimal answer on a number grid, follow these steps:

• Write the answer in the blank boxes across the top of the grid. Use a separate box for each number and for the decimal point.
• Below each number and decimal point, fill in the corresponding circle. The circles for decimal points are in the second row below the blank boxes.