All rows can be read from either side towards centre. The third symbol is divide by 3333. The result is 33 The Bad Threes. Nineapple Pineapple has found The Bad Threes in three steps down the Apple Tree Road.
According to Shakuntala Devi, there is no quick test to see if a number is divisible by 7
Figuring (The Joy of Numbers) and All in a Wonderland of Numbers by Shakuntala Devi
Mathemagic by Royal Vale Heath
Mathematics Magic and Mystery by Martin Gardner
Self-Working Number Magic by Karl Fulves
The Trachtenberg Speed System of Basic Mathematics