For Seniors: How to Use Common Excel Functions
Excel has hundreds of functions, but most of them are very specialized. The basic set of Excel functions that the average user works with is much more manageable.
The simplest functions have no arguments. Two prime examples are

NOW: Reports the current date and time.

TODAY: Reports the current date.
Even though neither uses any arguments, you still have to include the parentheses, so they look like this:
=NOW( )
=TODAY( )
Another basic kind of function performs a single, simple math operation and has a single argument that specifies what cell or range it operates on. The following table summarizes some important functions that work this way.
Function  What It Does  Example 

SUM  Sums the values in a range of cells.  =SUM(A1:A10) 
AVERAGE  Averages the values in a range of cells.  =AVERAGE(A1:A10) 
MIN  Provides the smallest number in a range of cells.  =MIN(A1:A10) 
MAX  Provides the largest number in a range of cells.  =MAX(A1:A10) 
COUNT  Counts the number of cells in the range that contain numeric values. 
=COUNT(A1:A10) 
COUNTA  Counts the number of empty cells in the range.  =COUNTA(A1:A10) 
COUNTBLANK  Counts the number of nonempty cells in the range.  =COUNTBLANK(A1:A10) 
The following table shows some functions that change how numbers are presented.
Function  What It Does  Example 

ABS  Presents the absolute value of the number.  =ABS(B1) 
ROUND  Rounds the number up or down by a specified number of decimal points. 
=ROUND(B1,0) 
EVEN  Rounds a positive number up, or a negative number down, to the next even whole number. 
=EVEN(B1) 
ODD  Rounds a positive number up, or a negative number down, to the next odd whole number. 
=ODD(B1) 
Here are some things to note about the items in the preceding table:

Absolute value: This is the positive version of a number. For example, the absolute value of –15 is 15. If the number is already positive, it stays positive.

Multiple arguments: See the entry for ROUND. There are two arguments: the cell address or number to operate on, and the number of decimal places. A value of 0 for the decimal places results in rounding to the nearest integer.
When a function has more than one argument, the arguments are separated by commas. You can see this in the ROUND example: ROUND(B1,0).
Two other functions, ROUNDUP and ROUNDDOWN, work just like ROUND except they specify which way the rounding will always occur.