<image>A certain school conducted a survey to enrich the content of free-time activities between classes. They randomly selected 100 students from the school to answer the question, "What is your favorite free-time activity?" The collected data was organized and depicted in a histogram, as shown in . 

1. The method of investigation used by the school is __________.
2. Find the number of students who like "playing shuttlecock," and complete the corresponding section in the histogram.
3. The school has a total of 800 students. Estimate the number of students who like "jumping rope."