Teacher:This task involves creating answers to complex questions, from a given passage. Answering these questions, typically involve understanding multiple sentences. Make sure that your answer has the same type as the "answer type" mentioned in input. The provided "answer type" can be of any of the following types: "span", "date", "number". A "span" answer is a continuous phrase taken directly from the passage or question. You can directly copy-paste the text from the passage or the question for span type answers. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words. A "number" type answer can include a digit specifying an actual value. For "date" type answers, use DD MM YYYY format e.g. 11 Jan 1992. If full date is not available in the passage you can write partial date such as 1992 or Jan 1992. If you find multiple spans, please add them all as a comma separated list. Please restrict each span to five words.
Teacher: Now, understand the problem? Solve this instance: Passage: Following their first tours in the early 1990s, Tool has performed as a headline act in world tours and major festivals such as Lollapalooza (1997 and 2009), Coachella Valley Music and Arts Festival (1999 and 2006), Voodoo Experience (2001 and 2016), Download Festival (2006), Roskilde Festival (2001 and 2006), Big Day Out (2007 and 2011), Bonnaroo (2007), All Points West Music & Arts Festival (2009), and Epicenter (music festival) (2009). They have been joined on stage by numerous artists such as Buzz Osborne and Scott Reeder (bassist) on several occasions; Tom Morello and Zack de la Rocha during their 1991 tour; Layne Staley in Hawaii, 1993; Tricky (musician), Robert Fripp, Mike Patton, Dave Lombardo, Brann Dailor of Mastodon (band), and experimental arts duo Osseus Labyrint during their 2001–02 Lateralus tour; and Kirk Hammett, Phil Campbell (musician), Serj Tankian, and Tom Morello during their 2006–07 tour. They have covered songs by Led Zeppelin, Ted Nugent, Peach (band), Kyuss, the Dead Kennedys, and the Ramones. 
Question: How many years after first performing at Voodoo Experience did Tool perform there again? 
Answer type: Number 
Student:
15.