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Khan Academy on a Stick

Pre-algebra

Conceptualizing decimals and place notation

You've been using decimals all of your life. When you pay $0.75 at a vending machine, 0.75 is a decimal. When you see the ratings of gymnastics judges at the Olympics ("9.5, 9.4, 7.5 (booooo)"), those are decimals. This tutorial will help you understand this powerful tool all the better. Before you know it, you'll be representing numbers that are in-between whole numbers all the time!

  • Regrouping with decimals

    We're doing more regrouping but this time using decimals. Just remember those place values!

  • Regrouping with decimals example 1

    Similar to a problem we did earlier but with whole numbers, this time we're having to regroup with decimals and find the right answer. Take your time.

  • Regrouping decimals example 2

    Great decimal regrouping exercise. Do this one with us and see how well you've mastered the regrouping decimal topic!

Regrouping decimals

Let's explore how we can regroup and redistribute value among the various place values in a decimal number.

Decimals on a number line

Let's think about where decimals are on a number line. It will help us understand what decimals represent in general!

Comparing decimals

Let's test our understanding of decimals by comparing them to one another!

Adding and subtracting decimals

You get the general idea of decimal is and what the digits in different places represent (place value). Now you're ready to do something with the decimals. Adding and subtracting is a good place to start. This will allow you to add your family's expenses to figure out if your little brother is laundering money (perhaps literally). Have fun!

  • Introduction to multiplying decimals

    Multiplying decimals can be confusing because there's always the question as to where the decimals goes in the answer. We're about to show you so you'll never have to question it again!

  • Multiplying decimals word problem

    We're multiplying decimals again, but this time answering a common question: what's the cost of filling up the car tank?

  • Multiplying decimals: place value and reordering

    We want you to develop an intuition about how to work with decimals. Understanding how you can rewrite decimals by considering the place value will help you multiply!

  • Multiplying decimals example

    Multiplying decimals? Try multiplying without the decimals first, them add them back in. We'll show you.

  • Multiplying challenging decimals

    Sometimes multiplying really small decimals (with all those zeros!) can be a little intimidating. Watch as we show you a handy trick to simplify these problems and solve them.

Multiplying decimals

The real world is seldom about whole numbers. If you precisely measure anything, you're likely to get a decimal. If you don't know how to multiply these decimals, then you won't be able to do all the powerful things that multiplication can do in the real world (figure out your commission as a robot possum salesperson, determining how much shag carpet you need for your secret lair, etc.).

Dividing decimals

You can add, subtract and multiply decimals. You know you'd feel a bit empty if you couldn't divide them as well. But something more powerful is going to happen. If you are like us, you never quite liked those pesky remainders when dividing whole numbers. Well, those pesky remainders better watch out because they are going to be divided too!!!! Ah ha ha ha ha!!!!!

Converting between fractions and decimals

Both fractions and decimals are desperate to capture that little part of our heart that desires to express non-whole numbers. But must we commit? Can't we have business in the front and party in the back (younger people should look up the word "mullet" to see a hair-style worth considering for your next trip to the barber)? Can't it look like a pump, but feel like a sneaker? Well, if 18-wheelers can turn into self-righteous robots, then why can't decimals and fractions turn into each other?

Intro to percentages

  • Growing by a percentage

    In this example we grow a whole number by a percentage of itself. Growing by percentage is a common skill often used when figuring how much is owed or earned with interest.

  • Solving percent problems

    We'll use algebra to solve this percent problem.

  • Percent word problem example 4

    This word problem is expressed simply which can be confusing to some. Take your time and focus on the what is being asked for.

  • Percent word problem example 5

    In this example, you working with us to find the number that is expressed as a given percentage.We'll create a simple algebraic equation to solve!

Percent word problems

Whether you're calculating a tip at your favorite restaurant or figuring out how many decades you'll be paying your student debt because of the interest, percents will show up again and again and again in your life. This tutorial will expose you to some of these problems before they show up in your actual life so you can handle them with ease (kind of like a vaccine for the brain). Enjoy.

Estimating and rounding with decimals

Laziness is usually considered a bad thing. But sometimes, it is useful to be lazy in a smart way. Why do a big, hairy calculation if you just need a rough estimate? Why keep track of 2.345609 when you only need 2.35? This tutorial will get you comfortable with sometimes being a little rough with numbers. By being able to round and estimate them, it'll only add one more tool to your toolkit.

Significant figures

There is a strong temptation in life to appear precise, even when you are aren't accurate. If you precisely measure one dimension of a carpet to be 3.256 meters and eyeball the other dimensional to be "roughly 2 meters", can you really claim that the area is 6.512 square meters (3.256 x 2)? Isn't that a little misleading? This tutorial gets us thinking about this conundrum and gives us the best practices that scientists and engineers use to not mislead each other.

  • Multiplying a decimal by a power of 10

    You will notice in this word problem that moving the decimal to the right the same number of times as the number of zeros you multiplying by gets you the answer you desire. Check this out!

  • Dividing a decimal by a power of 10

    When we were multiplying, we moved the decimal to the right for each power of ten. Guess what? When dividing, we move the decimal to the left for each power of ten.

  • Dividing a decimal by a power of 10: shortcut

    Holy cow! A shortcut? Yep. For each ten you multiply or divide by, just shift that decimal to the right or left. Watch this and be amazed.

Moving the decimal to multiply and divide by 10

In our decimal number system, as we move places to the left, the place values increase by a factor of 10 (likewise, they decrease by a factor of 10 as we move rightward). This idea gets direct application when we multiply or divide a decimal number by 10 because it has the effect of shifting every place value one to the right or left (sometime seen as moving the decimal point).

  • Adding, subtracting numbers in different formats

    In this example, we solve an addition problem with numbers in different formats: a percentage, decimal, and mixed number. First, we'll get them all in the same format...which makes it a while easier to solve!

  • Adding, subtracting fractions, decimals, percentages

    What happens when we are asked to add or subtract fractions, decimals, and percentages? You'll see in this example that we first get them in the same format, and then add fractions.

  • Adding, subtracting fractions

    Our focus here is on learning to multiply fractions. First we have to get the common denominator, then add positive and negative numbers.

Adding and subtracting decimals percentages and fractions

If you think you know how to convert between decimals, fractions and percentages and are familiar with negative numbers, this tutorial is a good place to test all of those skills at the same time.