USE GOAL SEEK

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Determine the value you require in a given cell.
Example: Total of 1000
Locate the cell Excel can change to achieve your result.
Example: C5

Begin
Display the Tools menu and then click the words Goal Seek. The Goal Seek dialog box will display.
Fill in the list boxes based on the value you require and the cell you can change.
Click the OK button. The Goal Seek Status dialog box will appear to let you know when Excel has found a solution.
Click OK to accept the solution.