This is an excerpt from Pierre Leclerc (www.excel-vba.com)
The 2 Most Important
Excel Functions AND
Lesson 19: Excel Analysis Toolpak
The Analysis Toolpak is an add-in created by Microsoft that adds 86 functions to the ones already available. When you activate the Analysis Toolpak (in versions of Excel earlier than 2007) you get the Engineering functions, many more Financial functions and a few Mathematical and Date and Time functions.
To activate the Excel Analysis toolpak go to "Tools/Add-ins" and check the box for Analysis Toolpak then "OK. From then on you really have 333 functions that are offered to you when you click on "Insert/Function" or when you click on the icon
COUNTIF ¦ SUMIF ¦ IFERROR