The number of people using an older version of a spreadsheet program decreases at a rate that is proportional at any time to the number of people still using the version at that time.
There were 10{,}00010,00010, comma, 000 people using the older version of the spreadsheet program when the new version first came out. The number of people still using the older version decreases by 20\%20%20, percent every 666 months.