Resources index

The Fantastic "Four"

This math tool is fantastic! With this one simple-to-use strategy, you can find the Greatest Common Factor (GCF), the Lowest Common Multiple (LCM) or Lowest Common Denominator (LCD), and if you use a numerator as the first number and a denominator as the second number, you can find equivalent fractions and the fraction in the simplest form!

Put your own numbers into each of the two boxes at the top of the "four". When you push the button, you will see the completed Fantastic Four worked out. You will also be given the GCF, LCM (or LCD), and the fraction in the simplest form.


How It Works

Of course, you will not always have a computer handy while you are doing your school assignments or homework. So you should learn to make this yourself. Luckily, it's pretty easy to make!

  1. Write a number 4 on your page. It should be the one with the open top. (I always put a capital "G" above the first column on the outside of the "four" to remind me where to find the GCF.)
  2. Write the two numbers, one in each space. To simplify a fraction, write the numerator first, the denominator second. To find the LCD of two fractions (so you can add or subtract them), use only the denominator of each fraction.
  3. Try to divide by primes. If both numbers can be divide by a prime number, then write that number in "front" of the other numbers (outside the "four" shape). Then divide each number and put the quotient underneath.
  4. Repeat step 3 with each new set of quotients until no other primes go into both.

That's all there is to it! Your GCF is the all the numbers in the first column multiplied together. The LCM/LCD is the GCF multiplied by the two numbers on the bottom row. If you were simplifying a fraction, you will find the simplified form on the bottom row, numerator and denominator in order.

BTW, you should already know the first five or six prime numbers, but in case you don't, here they are:

2, 3, 5, 7, 11, 13

TODO: trap error if button pushed with nothing in both fields.