Measure theory set construction I have a typical problem concerning the measure theory. I would like to know if any one has a good general strategy to solve this kind of problem because I always don't have idea where have I to begin to prove this kind of things.
The problem which I have to solve is:
If $C\subset \mathbb{R}$ is a lebesgue meas. set of measure $\lambda (C)=l>0$. To show is: $\forall 0<r<l \exists B\subset C$ with $\lambda (B)=r$.
Thanks for any answer

This is so called Serpinski's theorem. For the sketch of the proof for general non-atomic measures see this wikipedia article.
For your particular case, consider function $f(x)=\lambda(C\cap(-\infty,x))$. It is non-decreasing, so to prove continuity it is enough to prove continuity on the right and on the left of each point. But the latter is nothing more than so called continuity of measure. Since $f$ is continuous, the desired result follows from itermediate value theorem.
