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Khan Academy on a Stick

Pre-algebra

Divisibility tests

Whether you are trying to impress your dog's friends (who are obsessed with figuring out number divisibility) or quickly factor a number, it can be very useful to tell whether a number is divisible by another. This tutorial walks through some of the more standard divisibility methods and describes why they work.

  • Finding factors of a number

    To find the factors of a number means to find all the whole numbers that the number in question is divisible by. Can you help us find the factors in this example?

  • Finding factors and multiples

    Watch and listen as we explain a simple way of remembering the difference between factors and multiples.

  • Interactive module: finding factors

    Here's a video of Sal working on a exercise which helps you visualize the process of finding a number's factors.

Divisibility and factors

In this tutorial, we'll begin to think about the numbers that "make up" the number. This will be useful throughout our study of math. Whether we are adding fractions, exploring mystical number patterns, or breaking computer codes, factoring numbers are key! Eye of the tiger!

  • Prime numbers

    What does it mean to be a prime number? Let's progress though some whole numbers and ask ourselves if they meet the criteria. What is the criteria you ask? Watch.

  • Recognizing prime and composite numbers

    Can you recognize the prime numbers in this group of numbers? Which are prime, composite, or neither?

Prime numbers

Prime numbers have been studied by mathematicians and mystics for ages (seriously). They are both basic and mysterious. The more you explore them, the more you will realize that the universe is a fascinating place. This tutorial will introduce you to the magical world of prime numbers.

Prime factorization

You know what prime numbers are and how to identify them. In this tutorial, we'll see that *all* positive whole numbers can be broken down into products of prime numbers (In some way, prime numbers are the "atoms" of the number world that can be multiplied to create any other number). Besides being a fascinating idea, it is also extremely useful. Prime factorization can be used to decrypt encrypted information!

  • Least common multiple exercise

    We're practicing finding the least common multiple in these example exercises. We'll teach you the prime factorization approach which is a pretty nifty way to find the lcm (least common multiple).

  • Least common multiple exercise 2

    Here's a lcm (least common multiple) example exercise that's a little but harder, but totally doable. Again, we'll use the prime factorization approach.

  • Least common multiple exercise: 3 numbers

    This least common multiple example gives us 3 numbers from which to find the lcm. This is a challenge, but fun. Do it with us!

Least common multiple

Life is good, but it can always get better. Just imagine being able to find the smallest number that is a multiple of two other numbers! Other than making your life more fulfilling, it will allow you to do incredible things like adding fractions.

  • Greatest common factor exercise

    Find the biggest number that will divide into the given 2 numbers. This is called the GCF or greatest common factor.No worries, we'll show you how to do it.

  • Greatest common factor explained

    Here's a nice explanation of least common factor (or least common divisor) along with a few practice example exercises. Let's roll.

  • LCM and GCF word problems

    Here we have a couple of word problems--one searching for the least common multiple and the other for the greatest common factor. Just read them with us slowly and follow along. You'll get it.

Greatest common factor

You know how to find factors of a number. But what about factors that are common to two numbers? Even better, imagine the largest factors that are common to two numbers. I know. Too exciting!