If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

# Arithmetic with rational numbers FAQ

## Why do we need to line up the decimal points when we add or subtract decimals?

When we add or subtract decimals, we need to line up the decimal points to make sure we are adding or subtracting the same place values. For example, if we want to add 3, point, 4 and 0, point, 52, we need to write them like this:
\begin{aligned} &\phantom{+}3.4\\ &+0.52\\ \hline \end{aligned}
Notice how the decimal point is in the same column for both numbers. This helps us see that we are adding 4 tenths to 5 tenths, and 3 ones to 0 ones. If we don't line up the decimal points, we might get confused and add the wrong digits. For example, if we write them like this:
\begin{aligned} 3.4&\\ +0.52&\\ \hline \end{aligned}
We might think that we are adding 4 tenths to 2 tenths, and 3 ones to 5 ones, which would give us the wrong answer of 8, point, 6. So, lining up the decimal points helps us avoid mistakes and add or subtract decimals correctly.

## How do we divide whole numbers to get a decimal?

Sometimes, when we divide whole numbers, we get a remainder that is not zero. For example, if we divide 7 by 2, we get 3 as the quotient and 1 as the remainder. We write it like this:
7, divided by, 2, equals, 3 remainder 1
But what if we want to know the exact answer, without a remainder? We can use decimals to show the part of the dividend that is left over after dividing by the divisor. We do this by adding a decimal point and a zero to the dividend, and continuing to divide using place value. For example, we can write 7 as 7, point, 0 like this:
$\begin{array}{rr|rl} {2}&\overline{\Big)7.0}\\ &\mathllap{-}\underline{6.0}&\purpleD{3}\\ &1.0\\ &\mathllap{-}\underline{1.0}&\underline{{}+\purpleD{0.5}}\\ &\goldE{0}&\redE{3.5}\\ \end{array}$
Now we can divide the 1, point, 0 by the 2, and get 0, point, 5 (think 10 tenths divided by 2 is 5 tenths). So in all, we got a quotient of 3, plus, 0, point, 5, which equals 3, point, 5. We write it like this:
7, divided by, 2, equals, 3, point, 5
We can do this with any whole number division that has a remainder. For example, if we divide 9 by 4, we get 2 as the quotient and 1 as the remainder. We write it like this:
9, divided by, 4, equals, 2 remainder 1
To get the exact answer, we add a decimal point and two zeros to the dividend. We write it like this:
$\begin{array}{rr|rl} {4}&\overline{\Big)9.00}\\ &\mathllap{-}\underline{8.00}&\purpleD{2.00}\\ &1.00\\ &\mathllap{-}\underline{1.00}&\underline{{}+\purpleD{0.25}}\\ &\goldE{0.00}&\redE{2.25}\\ \end{array}$
Now we can divide the 1, point, 00 by the 4, and get 0, point, 25 (think 100 hundredths divided by 4 is 25 hundredths). So in all, we got a quotient of 2, plus, 0, point, 25, which equals 2, point, 25. We write it like this:
9, divided by, 4, equals, 2, point, 25

## How do we divide decimals by decimals?

To divide decimals by decimals, we need to make the divisor a whole number first. We do this by moving the decimal point in both the divisor and the dividend the same number of times to the right, until the divisor has no decimal point. This works because we are creating equivalent fractions.
For example, if we want to divide 3, point, 6 by 0, point, 4, we move the decimal point one time to the right in both numbers, and get 36, divided by, 4. We write it like this:
start fraction, 3, point, 6, divided by, 0, point, 4, end fraction, equals, start fraction, 36, divided by, 4, end fraction
Then we can divide as usual, and get 9 as the answer. We write it like this:
start fraction, 3, point, 6, divided by, 0, point, 4, end fraction, equals, 9
Another example is 0, point, 18, divided by, 0, point, 06. We move the decimal point two times to the right in both numbers, and get 18, divided by, 6. We write it like this:
start fraction, 0, point, 18, divided by, 0, point, 06, end fraction, equals, start fraction, 18, divided by, 6, end fraction
Then we can divide as usual, and get 3 as the answer. We write it like this:
start fraction, 0, point, 18, divided by, 0, point, 06, end fraction, equals, 3

## What is a reciprocal?

The reciprocal of a number answers the question, "How many groups of that number are in 1?"
Let's start with some basic numbers. How many groups of start fraction, 1, divided by, 3, end fraction are in 1? There are 3 groups of start fraction, 1, divided by, 3, end fraction in 1, so 3 (or start fraction, 3, divided by, 1, end fraction) is the reciprocal of start fraction, 1, divided by, 3, end fraction.
How many groups of 3 are in 1? There are no whole groups of 3 in 1, but there is a partial group of 3 in 1. Specifically, there is start fraction, 1, divided by, 3, end fraction of a group of 3 in the number 1.
Here are some of the ways this pair of reciprocal values relates.
\begin{aligned} 1\div \dfrac{1}{3} &= 3\\\\ 1\div 3 &= \dfrac{1}{3}\\\\ \dfrac{1}{3} \times 3 &=1 \end{aligned}
This pattern is true for all pairs of reciprocal numbers. Their product is always 1.
Let's use that fact to find the reciprocal of start fraction, 5, divided by, 2, end fraction.
\begin{aligned} \dfrac{5}{2} \times {?} &=1 \\\\ \dfrac{5}{2} \times {?} &=\dfrac{10}{10}\\\\ \dfrac{5}{2} \times \dfrac{2}{5} &=\dfrac{10}{10}\\\\ \dfrac{5}{2} \times \dfrac{2}{5} &=1\\\\ \end{aligned}
So the reciprocal of start fraction, 5, divided by, 2, end fraction is start fraction, 2, divided by, 5, end fraction and vice versa.
Notice a pattern? If we write a number in fraction form, then its reciprocal is a fraction with the numerator and the denominator flipped.

## Why do we divide fractions by multiplying by the reciprocal of the second fraction?

We often think of division as telling us how many groups of equal size we can make out of the total.
Suppose we want to know 5, divided by, start fraction, 2, divided by, 3, end fraction.
The reciprocal of a number answers the question, "How many groups of that number are in 1?" So the value of 1, divided by, start fraction, 2, divided by, 3, end fraction will be the reciprocal of start fraction, 2, divided by, 3, end fraction, which start fraction, 3, divided by, 2, end fraction.
Instead, we want to know 5, divided by, start fraction, 2, divided by, 3, end fraction, which is 5 times as much. So the quotient will also be 5 times as much as 1, divided by, start fraction, 2, divided by, 3, end fraction.
\begin{aligned} 5\div \dfrac{2}{3} &= 5 \times \left( \maroonD{1 \div \dfrac{2}{3}}\right)\\\\ &= 5 \times \maroonD{\dfrac{3}{2}} \end{aligned}
Thus, 5, times, start fraction, 3, divided by, 2, end fraction has the same value as 5, divided by, start fraction, 2, divided by, 3, end fraction.

## When do we perform operations on rational numbers in real life?

There are many situations in real life where we might need to perform operations on fractions and decimals. For example, when we're cooking, we might need to multiply or divide fractions when measuring ingredients. If a recipe calls for start fraction, 3, divided by, 4, end fraction of a cup of flour, but we only have a start fraction, 1, divided by, 2, end fraction cup measure, we would need to divide start fraction, 3, divided by, 4, end fraction, divided by, start fraction, 1, divided by, 2, end fraction to know how many times to fill the measure to get the right amount.
We also encounter decimals when we're dealing with money. We might need to add or subtract amounts to calculate a total bill or to make change. For example, if we're buying two items that cost dollar sign, 2, point, 59 and dollar sign, 3, point, 99, we need to add 2, point, 59, plus, 3, point, 99 to get a total of dollar sign, 6, point, 58.
There are countless other situations where we might use fractions or decimals in everyday life – when we're cutting things into portions, calculating percentages, or dealing with measurements, just to name a few.