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

Okay let's say I have 1/2 and 2/4 (this is really easy) so you take the multiples of the denominator and the numbers that are the same are known as common denominators so its

3 6 9 12 15 18 21 24

7 14 21 28

so that means 21 is the common denominator but there are way more than that trust me so now hopefully you guys get it

The common denominator is 21. So 1/3 would become 7/21 and 2/7 would become 6/21.