And adding numbers is sometimes simpler than subtracting numbers. It might be faster than my other method, because it is faster to get to a recognizable multiple of 7 ( a number between 0 and 70) by adding multiples of 5 than subtracting multiples of 2. This has come to be called “Chika’s Test”.
Continue… if the result is divisible by 7, then the original was divisible by 7. Chop off units’ digit and multiply by 5. Īnother version of a divisibility test for 7s was discovered** by Chika Ofili, a 12-year-old Nigerian boy, in 2019. If the final number is divisible by 7, then the original number was divisible by 7. Continue this process until the result is 1 or 2 digits. Subtract this from the remaining number. Chop off the units’ digit and double it. This is a useful fact for KenKen and Kakuro puzzles! Other decompositions of 3 addends all include repeated numbers: 3 + 1 + 3 or 2 + 3 + 2 or 1 + 5 + 1.ĭivisibility* by 7 is something I learned when doing more advanced KenKen puzzles. 
There is only one way that 7 can be decomposed into 3 unique addends, and they are consecutive multiples of 2: 1 + 2 + 4 = 7.
Sadly 187 breaks the sequence since it is a multiple of 11 and 17.
7 is the start of an arithmetic progression of six prime numbers: 7, 37, 67, 97, 127, 157. 7 is the number of days in a week, 14 is the number of days in a fortnight, and 28 is the number of days in a lunar month. so I’m calling it “Sevens Day”! Here are some wonderful things about the number SEVEN and related numbers: Today is July 14, 2021, which can be written 7-14-21….
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