I have this question as a review question:

Question

The reaction N2(g)+3H2(g)<-->2NH3(g) begins with 1.15 M N2 and 1.36 M H2. At equilibrium, the concentration of NH3 is 0.78 M.

Determine the extent of reaction.

The answer is supposed to be 86 but I don't know how

Answer 1

Here is a site you can read. I don't think 86 is the right answer either.
I think closer to 0.39 (39%)
https://en.wikipedia.org/wiki/Extent_of_reaction

Answer 2

ok thanks!