When to use MC/I and when to use MC/It?

[Sheogoroth]

I'm in a Strength of Materials course and I'm confused about when to use Mc/I and when to leave off the t, and what exactly the t and the c are. I thought that t was the distance to the centroid and that c was the distance to the line of action, but I'm running through practice problems and I think it's wrong.

 

[Jupiter6]

c is the distance from the edge to the centroid of the beam, yes. t is the particular point you want to look at in the cross section of the beam. In max tension or compression stress, t=c. Otherwise it becomes a ratio.