A Guard is stationed at a secret club.
His new club is very exclusive so he came up with his own secretive code so members could access the building he is Guarding with no hassle.
One member walks up and the Guard says, “10”. The man responds “11” and is let through.
Another man walks up and the Guard says, “25”. The man responds “23” and is also let through.
A third man walks up and the Guard says again “27” to which the man responds “29” and it let through.
You walk up to the Guard who says “78”. What Number Do You Say So The Guard Lets You In?