Given only the number of users in 2005 and the fact that it triples each year, how did you determine the number of users in 2004?

The question tells us that the number of users triples each year, right?  So for any given year (we don't have to know which one), the number of users the next year will be triple the number of users that year [note: I like to re-word problems until I'm sure that I understand them and can solve them]. So whatever (let's say, x) the number of users in 2004, the number of users in 2005 will be triple that (or, 3x).  Ah-ha ! the problem tells that amount, so set them equal:        3x = 54 million users in 2005          [note: I made that up]         x = 18 million users in 2004      [divide both sides by 3]and,       3(3x) = 162 million users in 2006    [3*what it was in 2005] Given any year, this is how you go forward or backward to find the number of users:  write the equation !  Then solve it.