classes:  lecturer. It takes about 15 - 20 minutes to open the slide and read the slide, surfing very quickly so students can not understand the lesson.  curriculum. It takes about 15 - 20 minutes to open the slide and read the slide, surfing very quickly so students can not understand the lesson.  facility. It takes about 15 - 20 minutes to open the slide and read the slide, surfing very quickly so students can not understand the lesson.  general. It takes about 15 - 20 minutes to open the slide and read the slide, surfing very quickly so students can not understand the lesson.