All right, you look at the slope on the oscilloscope. You find that the 20% point and the 80% point are 4.6 divisions apart horizontally. So how long did the signal take to rise from its 20% point to 80% point?
That's it. You have just computed the 20% to 80% rise time. End of story.
I will not post in this thread anymore, lest the others get jealous in post counts. If you still do not understand,