In Genesis 41, Joseph interpreted Pharoah's dreams as predicting 7 years of plenty followed by 7 years of famine. In verse 34, he recommends a 20% tax on grain during the good years to provide food during the "bad" years:

Let Pharaoh appoint commissioners over the land to take a fifth of the harvest of Egypt during the seven years of abundance.

How was this enough to "cover" all 7 years of famine (especially given the fact that they were evidently supplying grain to other parts of the Ancient Near East, given that Joseph's brothers eventually went there to get grain)? Also, if the grain was stored, the grain would've needed to last a minimum of 8 years. How would they have made it last that long? Is there any possibility that they would've planted at least some of the grain that they had collected, for example?