Tanner and Marco are making presentations for a class project. Tanner's slideshow starts with a verbal introduction that is 21 seconds long, and then each slide is left up for 10 seconds. Marco leaves each slide onscreen for 11 seconds, and his introduction lasts 11 seconds. Tanner and Marco notice that their presentations have both the same number of slides and the same duration. How long is each presentation? How many slides are in each presentation?
Let x be the number of slides in each presentation.
Tanner's presentation lasts for 21 + 10x seconds.
Marco's presentation lasts for 11 + 11x seconds.
Thus, 21 + 10x = 11 + 11x.
Therefore, x = 10.
Thus, each presentation has 10 slides.
So each presentation lasts for 21 + 10(10) = 21 + 100 = <<21+10*10=121>>121 seconds. Answer: \boxed{121}.