> # Oskar Hagberg 2012-03-12
> # The standard deviation is 
> sdev = 3
> # The length of a 95 % confidence interval is
> # 2*1.96*3/sqrt(n)
> # The problem can be solved by simple trial and error:
> 2*1.96*3/sqrt(120:139)
 [1] 1.0735362 1.0690909 1.0647004 1.0603635 1.0560792 1.0518464 1.0476641 1.0435313 1.0394470
[10] 1.0354103 1.0314202 1.0274760 1.0235766 1.0197213 1.0159093 1.0121396 1.0084117 1.0047246
[19] 1.0010777 0.9974702
> # Hence 139 is the minimum.
> # Alternative solution: solve
> 
> # 2*1.96*3/sqrt(n) <1
> # n > (2*1.96*3)^2
> (2*1.96*3)^2
[1] 138.2976