If I understand you properly, you are trying to set up a formula that will calculate how many hours you have worked during the day, "less" the time you spent out to lunch. In which case the formula needs an "out/in" function, not an "in/out" function.
Look at it this way. You have 5 columns, with 5 headers, A5 - B5 - C5 - D5 and E5.
The headers are;
Start time - Begin Lunch - End lunch - End time - Total worked.
In A7 is 08:00
In B7 is 12:00
In C7 is 13:00
In D7 is 17:00
In E7 is the result of the calculation giving you the time you worked, "less" the time you spent at lunch.
In the formula you gave, (C7<B7) always fails because C7 can never be smaller than B7. You cannot end your lunch before you begin it.
Also, the * 24 does not give any meaningful answer. It multiplies any result by 24, but what you want, (if I understand this correctly), is how many hours you worked during the day.
So, you need to take the End time and subtract the Start time, then from the result, take away the time spent at lunch, (which will be the End lunch minus the Begin lunch).
ie, you need D7-A7, gives total time at work, less (C7-B7), gives the time you worked, less your lunch time.
eg E7 =(D7-A7-(C7-B7))
Is this what you mean?
Incidently, each cell needs formatting, under the Numbers tab, to "Number" with 2 decimal places.
I'm using this formula: =(C7-B7+(C7<B7))*24
Of course the columns and rows vary. I want to have a column for lunch. I want a column for the time spent at lunch. Then have the total daily presented with lunch subtracted.