Here's how to subtract 3/12 from 7/9:


Step 1We can't subtract two fractions with different denominators. So you need to get a common denominator. To do this, you'll multiply the denominators times each other... but the numerators have to change, too. They get multiplied by the other term's denominator. So we multiply 7 by 12, and get 84. Then we multiply 3 by 9, and get 27. Next we give both terms new denominators  9 × 12 = 108. So now our fractions look like this:


Step 2Since our denominators match, we can subtract the numerators. 84 − 27 = 57 So the answer is:


Step 3Last of all, we need to simplify the fraction, if possible. Can it be reduced to a simpler fraction? To find out, we try dividing it by 2... Nope! So now we try the next greatest prime number, 3... Are both the numerator and the denominator evenly divisible by 3? Yes! So we reduce it:
Let's try dividing by 3 again... Nope! So now we try the next greatest prime number, 5... Nope! So now we try the next greatest prime number, 7... Nope! So now we try the next greatest prime number, 11... Nope! So now we try the next greatest prime number, 13... Nope! So now we try the next greatest prime number, 17... Nope! So now we try the next greatest prime number, 19... Nope! So now we try the next greatest prime number, 23... No good. 23 is larger than 19. So we're done reducing. There you have it! The final answer is:
