There is a maximum limit on the total yearly employee pre-tax or Roth salary deferral into the plan. This limit, known as the "402(g) limit", was $15,500 for the year 2008; $16,500 for 2009-2011; $17,000 for 2012; $17,500 for 2013-2014; $18,000 for 2015-2017; and $18,500 for 2018. For future years, the limit may be indexed for inflation, increasing in increments of $500. Employees who are at least 50 years old at any time during the year are now allowed additional pre-tax "catch up" contributions of up to $5,000 for 2008; $5,500 for 2009-2014; and $6,000 for 2015-2018. The limit for future "catch up" contributions may also be adjusted for inflation in increments of $500. In eligible plans, employees can elect to contribute on a pre-tax basis or as a Roth 401(k) contribution, or a combination of the two, but the total of those two contributions amounts must not exceed the contribution limit in a single calendar year.  This limit does not apply to post-tax non-Roth elections.

Answer this question based on the article: How many years was the pretax for $6000 for 2015-2018?
3