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This is one of the 33 lessons from the
Tutorial on Excel

Lesson 3-5: ISERROR Function in Excel

Sometimes a formula will return an error message like #DIV/0, #NA and others. There is one way to avoid such results and it is by protecting you formulas with the ISERROR function. For example the formula =B1/A1 will return a #DIV/0 if A1 is empty or equal to zero. The formula will then look like this:
=IF(ISERROR(B1/A1),0,B1/A1)
In plain English: should the result of A1 divided by B1 be an error change the result to 0 else show the result of B1/A1.

The is a somewhat easy approach to do this specially if you are working with long formulas. First develop your formula without the ISERROR protection =B1/A1  then follow these steps. 
insert between the equal sign and the original formula this piece of formula:
  =IF(ISERROR(),0,9)B1/A1
Then copy the original formula at two places between the parentheses of the ISERROR condition and replacing the 9:
=IF(ISERROR(B1/A1  ),0,B1/A1  )B1/A1
Finally remove the original formula at the end of the new one.
  =IF(ISERROR(B1/A1  ),0,B1/A1  )

This is one of the 33 lessons from the
Tutorial on Excel