What is the denominator between 1/3 and 2/7

I find it's easiest to find the middle of numbers with a simple trick I learned just today!

First, you find out how much the distance is between the two numbers. 

0-1-2-3-4-5-6-7

If you count, there is a distance of four in between 3 and 7. So we do the next step. 

In the next step, all you need to do is split the distance of the numbers in half. Then we can subtract half fro the bigger number or add the half to the smaller half. 

4 / 2 = 2
7 - 2= 5, or 3 + 2 = 5

So the denominator between 1/3 and 2/7 is 0/5. You can add any number you want to the top I think.