Something is known, but known to be missing. It is like an empty seat at the dinner table and you know who is not there.
The 5 is often used at the beginning of a communication to ask a question. 'What is your name? 'becomes: 'Your name is absent in my mind.'
 

• 53 who?
• 59 when?
• 52 how?
• 51 where?
• 577 what is going on?
• 56 what is the outcome?
• 58 what are the benefits?
• 54 what are the negatives?
• 525 what is the problem? ('why is the intended effect absent?')

 
When the 5 is used after another number the question aspect is lost and the 5 means that something is indeed absent or missing.

• 95 past tense; history
• 25 impossible; cannot be done-or it seems so
• 235 automatic ('process with no people')
• 45 no faults; debugged
• 55 don't know what we don't know.