+17 A new acne medication guarantees a 16 percent decrease in acne every week it is used. If a teenager began with 50 pimples, how many weeks before he was down to fewer than 5 pimples?
+129933 Let's see how Spawn got that...
16% decrease means that each successive week, the number is 84% of the that of the previous week....so we have
50(.84)^N < 5 where N is the number of weeks .....divide both dises by 50
(.84)^N < .10 take the log of both sides
log (.84)^N < log ( .10) and we can write
N log(.84) < log (.10) divide both sides by log (.84).....notice....since this is a negative quantity, we must reverse the inequality sign
N > log(.10) / log(.84)
N > 13.2
But....since the answer must be in whole units, N = 14
