Is there any chance that {text_1} is null? If so, you need to do one or more of the following things:
1. In the Formula Editor, look for "Exceptions for nulls" in the button bars at the top of the screen. Change it to "Default values for nulls".
2. Go to the File menu and select Report Options and turn on "Convert NULL values to default".
3. Handle it the old-fashioned way by checking for null in your formula - something like this:
If IsNull({text_1}) or not NumericText({text_1}) then 0
else ToNumber(Left({text_1},InStr(1,1},"/")-1))
NOTE: If using method 3 you ALWAYS have to test for NULL first!
-Dell