It you select the blank cell under a column of numbers or a blank cell at the end of a row of numbers and hit the Autosum button it will enter the SUM function and select the cells above in the column, or to the left in a row. Pressing Alt and the = key will also do the same thing. If you select the column or the row with the numbers and click the button or do Alt and the = key, then it will also do the same.
Sum Function
The Sum function or the AutoSum tool.
In Excel you can use the COUNT function to count the amount of cells that have numbers in them and the COUNTA function to count cells that have any kind of data in them.
Normal style
To be technically accurate, no function does this. The answer you are looking for is the AVERAGE function. It divides by the amount of cells that have values in them, not by the amount of cells. In most situations, all of the selected cells have values in them, but there are cases when they don't.
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A function can reference cells or named ranges in the function.
The Average function can use numbers, cells or ranges or a mix of these. IT can take up to 255 values. The general syntax is: =AVERAGE(number1, [number2], ...)
In Excel is it COUNT.
The COUNT function is designed to just count the amount of numbers that are in cells. There are lots of situations where you want to count only the cells that have numbers. You will often have ranges that have some empty cells and you want to only know what cells have numbers in them. There are other Count functions like COUNTBLANK that will count just the blank cells.
You can use Conditional Formatting to achieve this.
Count counts the amount of values that are in cells. If you have a block of cells of which some have numbers and some don't, it will tell you how many have numbers. It counts cells with numbers, dates and times, but not text or logical values. To do those you need the Counta function. To count the amount of values in the cells from B2 to B20 you would do this: =COUNT(B2:B20)