Dennis says that 1 1/2,1 2/4, and 1 3/6 are all equivalent. Is he
correct?

Yes

Step-by-step explanation:

Most fractions can be simplified into smaller fractions, while some are in it's simplest form. When putting a bit in its simplest form it is much easier to compare to other fractions  

What I did first tried to simplify all the factions, starting with 1 1/2.

After looking at 1 1/2, I can see that it is in its simplest form.

Then looking at 1 2/4 I can see that it can be simplified into 1 1/2.

Lastly looking at 1 3/6 I can see that it can be simplified into 1 1/2.