Question 515374
What we have to do in this problem is find the LCD, or Least common denominator. To do this, you can multiply the denominators:

5 x 7 x 3 = 105

This does not always work, but in this case it does.
We now must convert all our fractions. To do this the process is simple. Lets start with the 1/5 (one fifth) part of 2 and 1/5.

Divide 105 by 5.

105 / 5 = 21

So, we multiply the 1 in one fifth by 21.

1 x 21 = 21

And get 21/105 (twenty-one one hundred and fifths).




Lets do this process for the other two fractions.

105 / 7 = 15

15/105 (fifteen one hundred and fifths).



105 / 3 = 35

35/105 (thirty-five one hundred and fifths).

Now it is simple.

2 21/105 (2 1/5) <  2 35/105 (2 1/3) <  3 15/105 (3 1/7)

Hope I helped, all the best.
-mucklef