Here's how to subtract 1/7 from 3/8:


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 3 by 7, and get 21. Then we multiply 1 by 8, and get 8. Next we give both terms new denominators  8 × 7 = 56. So now our fractions look like this:


Step 2Since our denominators match, we can subtract the numerators. 21 − 8 = 13 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... 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... No good. 17 is larger than 13. So we're done reducing. There you have it! The final answer is:
