$(1-\frac12)\times(1-\frac23)=\frac12\times\frac13=\frac16$. 
I ate half of an apple pie on Saturday and two-thirds of the remainder on Sunday. What fraction of the pie was left for Monday? 
Options:
1) None 
2) $\frac12$ 
3) $\frac13$ 
4) $\frac23$ 
5) $\frac16$ 