The WTO was established on Jan 1, 1995. In july 2008, 153 countries were members of the organization representing 95% of the total world trade. If this rate continued, what year does the WTO rate actually represent 100%?

Is the rate linear or exponential?

If linear, it will take 20/19 of the 13.5 years so far
If exponential, it will take 13.5^(20/19) years