Hi--
is the set {1/n | n \in N} dense in [0,1].
simply ask yourself the following:
is every number in [0,1] either a limit point of the given set or in it?
if yes, then you have a dense set in [0,1]
else, you don't
in this case, the example given (3/4) does not satisfy any of the two conditions. (why?)
Conclude.