Function: Type this   but first click once in the cell where you want the result to go:
Addition =X1+X2+X3
Subtraction =X1-X2
Multiplication =X1*X2
Division =X1/X2
Sum =SUM(X1:X14)
Sheets =Sheet1!X2     (place this formula in a cell on a different sheet. You can have more than one sheet in an Excel file.)
Sum using Sheets =Sheet1!X1+Sheet2!X1+Sheet3!X1
Sum for many Sheets =Sum('Sheet1:Sheet9'!X1)
  where X1, X2, and X3  represent, for example, the cells A1, A2, and A3 or any other cells. (You must type the actual cell number, such as B12 or D4 and not "X1," "X2," etc.)