Write a system of equations to describe the situation below, solve using substitution, and fill in the blanks. Zachary and Valentina are making presentations for a class project. Zachary's slideshow starts with a verbal introduction that is 15 seconds long, and then each slide is left up for 11 seconds. Valentina leaves each slide onscreen for 8 seconds, and her introduction lasts 18 seconds. Zachary and Valentina notice that their presentations have both the same number of slides and the same duration. How many slides are in each presentation? How long is each presentation?