Decimals are numbers written using the base-ten place-value system. A very visible use of decimals is money. The chart below shows the names of the place values. Each place value is 10 times the value to its right. The decimal point is usually read as “and.”

1,004,307.6 is one million, four thousand, three hundred seven and six tenths
60.543 is sixty and five hundred forty three thousandths

Each digit of a number has a name based on place value. 1 = units digit 10 = tens digit 100 = hundreds digit 1,000 = thousands digit

In the number

34,567

3 is the ten-thousands digit 4 is the thousands digit 5 is the hundreds digit 6 is the tens digit 7 is the units digits

The GRE often asks questions using digits.

Example one

Show the value of a number with x as the units digit, y as the tens digit, and z as the hundreds digit.

The GRE can give puzzle problems. Use the strategy of reading the entire question before you start.

Example two

A number has two digits to the left of the decimal point and two digits to the right of the decimal point. The hundredths digit is two times the tenths digit. The number has 2 as the tenths digit. The ones digit is greater than zero and less than the tenths digit. When 1 is subtracted from the tens digit, the result is 5. What is the number?

Draw a quick picture of the number. ____ ____ . ____ ____

A sentence in the middle of the question starts the process: the tenths’ digit is 2.
The next sentence uses the ones digit, so find that. The ones digit is the only whole number between 0 and 2, so is 1.
The tens digit minus 1 equals 5, so the tens digit is 6.
Then back to the second sentence. The hundreds digit is two times the tenths digit, 2 × 2 = 4.

So the answer is 61.24.

Rounding

Rounding can be used to get a value that’s easier to use in calculations, or to get an estimate for the answer. For example, if you are buying a shirt for $19.99 and pants for $28.49, a quick estimate of the total is $20 + $28 = $48.

To round a decimal to a given place value, look at the digit in the place to the right.

If the digit is less than 5, round down.

If the digit is 5 or greater, round up.

Using a number line helps visualize rounding. Round to the value closest to the number.

Example one

Round each number to the nearest unit and nearest tenth.

6.25 is closer to 6 than 7, so it rounds to 6 as the nearest unit. Since the digit to the right is 5, 6.25 rounds to 6.3 as the nearest tenth.

Since the tenths digit is 5, 8.5 rounds up to 9, and remains 8.5 for the tenths.

9.846 rounds to 10 and to 9.8. Do not round up from 9.846 to 9.85 before rounding to the tenths’ place. (If you did you would get 9.9, which is incorrect.)

Decimals and Fractions

Decimals can be used to write fractions. If the fraction has a denominator that is a multiple of ten, and the numerator does not end in 0, then the number of zeros in the denominator will be the number of places to the right of the decimal.

(a) 3/100 = 0.03 The denominator has 2 zeros, so there are 2 digits to the right of the decimal point. You need to add a zero.

(b) 23/1000 = 0.023 The denominator has 3 zeros, so there are 3 digits to the right of the decimal point. You need to add a zero.

(c) 45/10 = 4.5 The denominator has 1 zero, so there is just 1 digit to the right of the decimal point.

Note: The leading zero, before the decimal point, is there to make the decimal more visible: 0.35 = .35, but the zero makes easier to see the decimal point and not confuse 0.35 with 35.

Decimals can be used to write other fractions. Use the fraction bar as a division symbol.

There are 3 digits to the right of the decimal point, so the denominator under 75 has 3 zeros.

(c) 31.2 = 31\dfrac{2}{10} = 31\dfrac{1}{5}

The answer could also be an improper fraction.

31.2 = 31\dfrac{1}{5} = 156/5

Adding and Subtracting Decimals

To add or subtract decimals, use a vertical format and line up the decimals. You can add trailing zeros as placeholders at the end of a decimal to make the alignment easier. Be sure to write the decimal point in the answer.

The simple way to solve this problem is to do the addition then the subtraction.

Find the sum.

6.980 3.217 \dfrac{+\,4.000}{14.197}

Subtract.

14.197 \dfrac{-\,3.637}{10.560}

Multiplying and Dividing Decimals

Multiply two decimals just like you would multiply two integers, then place the decimal point in the answer. If both factors have decimals, the number of decimal places in the product is equal to the sum of number of the decimal places in the two factors.

There are 2 decimal places in 3.02 and none in 6, so there will be 2 decimal places in the product.

6 × 3.02 = 18.12

You can check your answer using rounding. 6 × 3 = 18, so your answer is reasonable.

(b) Multiply as integers. 4 × 6 = 24

There is 1 decimal place in 0.4 and 2 decimal places in 0.06, so count in 3 decimal places from the right. This means you need to add a zero after the decimal point, to have the 3 decimal places.

0.4 × 0.06 = 0.024

To divide decimals, move the decimal point in the divisor to the right so it’s a whole number. Then move the decimal point in the dividend the same number of places.

Keep the decimal points aligned in the dividend and the quotient.

You can check the decimal point placement in your answer by looking at the multiplication. 4.5 × 9.5 will have 2 decimal places in the answer, and 42.75 has 2 decimal places.

Move the decimal point in 0.02 two decimal places to get the whole number 2. To move the decimal place in 50 by 2 places, you need to add 2 zeros to get 5,000.