Excel provides a handy graphical tool for entering formulas called the Calculator.
To build a formula with the Calculator:
1.  Click in the cell where you want the formula (Figure 11.31). Figure 11.31. Select the cell where you want the formula to appear.

2.  Choose Tools > Calculator. The Calculator appears, and the formula is already started for you in the main window of the Calculator (note the equal sign) (Figure 11.32). Figure 11.32. The calculator appears.

3.  Click the buttons to build the formula. When the formula can be evaluated into a number, that number appears in the Answer box. 
4.  If the formula you want to create involves a sum, click the Sum button and the calculator extends a pane to the right (Figure 11.33). Click and drag through the range of cells to sum, and click Insert to place the formula in the formula pane on the left. If that's all you want in your formula, click OK to place it in the selected cell (Figure 11.34). Figure 11.33. The pane for the Sum function appears.
Figure 11.34. The formula is calculated in the selected cell.

5.  If you want to use a different function, click the More button. The Paste Function window appears with the complete list of available functions divided into categories on the left; clicking on a category shows the list of functions available in that category (Figure 11.35). Choose the function you want, and click OK. Figure 11.35. Function categories appear on the left; functions in the selected category are on the right.

6.  Another window appears, prompting you for the appropriate arguments for the function (Figure 11.36). For many common functions, Excel will guess the cells you want to use as parameters. If it guesses wrong, you can click on or click and drag through the correct cells, and click OK to include the function in the formula pane of the calculator. Figure 11.36. Enter the information needed by the function.

7.  You may even set up a condition so that the formula is different depending on whether the condition is true or false. Click the If button to set it up; a pane extends to the right where you specify the condition and results (Figure 11.37). When the condition and results are defined, click Insert and they appear in the formula pane in the calculator (Figure 11.38). Figure 11.37. The conditional pane appears.
Figure 11.38. The formula with the condition appears in the Calculator.

8.  When the formula looks the way you want, click OK and it appears in the designated cell (Figure 11.39). Figure 11.39. The formula appears in the selected cell.
