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grammy:

the percentage of people age 18-24 who smoke has dropped from 5.2% in 2001 to 3.4% in 2004. Calculate the percentage of people age 18-24 who smoked in 2003 and estimate the percentage of people age18-24 who smoked in 2008

the only (main) really confusing part of this question, for you i mean, is that the percentages quoted have got nought to do with the percent reduction of smokers. if you change the percentages given to decimals that may help a little.

S = P(1-r) ⁿ
P is the initial percentage = 5.2% = 0.052
S is the future percentage
n is the number of years since 2001
r is the yearly percent decrease in smokers percentage

0.034 = 0.052(1-r) ³

0.653846 = (1-r) ³

0.653846 ^1/3 = 1-r

r = 1 - 0.653846 ^1/3

r = 0.132

so the percentage of smokers is decresing by an aveerage of 13.2% per annum

S = 0.052 * (1-0.132) ⁿ
S = 0.052 * 0.868 ⁿ

For 2003, n=2
S = 0.052 * 0.868 ²
= 0.039
that is, in 2003, 3.9% of the age group smoked.
Always check that your answer is sreasonable. This is between the 2001 and the 2004 values so sounds reasonable

You can do 2008 yourself.