The equation which calculates the percent of the original wieght of the falling rock that is lost is
weight% =100*.5^time where time is measured in minutes.
Given that weight% = 100/3
100/3 =100*.5^time
1/3=.5^time
log(1/3) =time*log(.5)
time=log(1/3)/log(.5)
time=(-.477)/(-.301)             <-------- aproximation
time=1.6                                <--------aproximation
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