Test of divisibility for prime numbers
Rule 1:
A number is divisible by 7 if and only if the number of tens subtracted to 2 times the number of units is also divisible by 7.
Examples:
343 is divisible by 7 since 34 – 2 x 3 = 28, which is divisible by 7
1197 is divisible by 7 since 119 – 2 x 7 = 105 and 105 is divisible by 7 since 10 + 5 5 = 10 + 25 = 35 Which is divisible by 7
Rule 2:
A number is divisible by 7 if and only if the number of tens added to 5 times the number of units is divisible by 7.
105 is divisible by 7 since 10 + 5 5 = 10 + 25 = 35 Which is divisible by 7
3192 is divisible by 7 since 319 + 5 2 = 319 + 10 = 329 and 32 + 5 9 = 32 + 45 = 77 Which is divisible by 7.
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Rule 1:
A number is divisible by 7 if and only if the number of tens subtracted to 2 times the number of units is also divisible by 7.
Examples:
343 is divisible by 7 since 34 – 2 x 3 = 28, which is divisible by 7
1197 is divisible by 7 since 119 – 2 x 7 = 105 and 105 is divisible by 7 since 10 + 5 5 = 10 + 25 = 35 Which is divisible by 7
Rule 2:
A number is divisible by 7 if and only if the number of tens added to 5 times the number of units is divisible by 7.
105 is divisible by 7 since 10 + 5 5 = 10 + 25 = 35 Which is divisible by 7
3192 is divisible by 7 since 319 + 5 2 = 319 + 10 = 329 and 32 + 5 9 = 32 + 45 = 77 Which is divisible by 7.
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