You have to find a common number divisible by both numbers of the fraction until you cannot divide the numbers of the fraction any more. So on your problem 18/42 start by dividing both numbers by say 2 18 divided by 2 gives you 9 42 divided by to gives you 21 Then try dividing them again say by 3 9 divided by 3 gives you 3 21 divided by 3 gives you 7 If you try dividing again you will see that you can no longer divide it so 3/7 would be your answer. I hope that helps.