I purchased a 25 gm active dry yeast. It says to use it with 500 ml flour. However I just want to make a couple of pizza bases, 100 gm each max. So can I use 40% of it today, and use the rest later? How to store the open pack, do I need to seal it and pack?
Yes, you can store an opened pack of dried yeast without a problem. Seal the pack as well as you can (plastic clips for just that purpose are very common, or you can just use an office 'bulldog' clip), and put it in the fridge.
I keep my active dry yeast in freezer (aprox -10C) to avoid humidity and fungus as freezers have higher dehydration than fridge (1-7C). The same is not valid for the liquid yeast. Freezer will kill the bacteria.