Inititally 100
after one minute 50 left
after 2 minutes 25 bubbles left
after 3 minutes 2 bubles left
after 4 minutes 0 bubles left
the question is unclear as to whether the bubbles are blown at the end or at the beginning of each minute.
Let's say they are blown at the end of each minute.
So at (immediately after) the end of ten minutes there are
100 bubbles that have just been blown
+ 50 that were blown a minute ago
+ 25 that were blown 2 minutes ago
+ 2 that were blown 3 minutes ago
and all the ones that were blown before that are gone.
You can add them up :)