are there any rules of divisibility by 7 and 8

Question

are there any rules of divisibility by 7 and 8?

Zoelef · Answer

Google is your friend here. ^^ 

Dividing by 8: If the last 3 digits are divisible by 8, so is the entire number. (Ex: 6,008 is divisible by 8 since 008 is divisible by 8. 6,018 is not since 18/8 has a remainder.) 

Dividing by 7: There are formulas, but they appeared too complex to explain concisely, and given their complexity I can't imagine they'd be that vital to GMAT performance.

silvelho · Answer

Divisibility by 7: 1 - Eliminate the last two digits of N; 2 - Calculate the difference between the number formed by the last two digits of N and the immediate superior multiple of 7. Add the result to the digit of the thousands (if necessary, subtract the immediate inferior multiple of 7 from the sum). Repeat the procedure until the leftmost pair of digits is reached. If the leftmost pair of digits is incomplete, consider the lacking digit as a zero. If the final result is a multiple of seven then the tested number is also a multiple of seven.
Example: N = 6,324,612
Using common language: 12 to 14 = 2; 2 + 4 = 6; 66 to 70 = 4; 4 + 3 = 7; 72 to 77 = 5; 5 + 0 = 5 → 56; 7|56 and 7|N
Please, if you want to watch how quick is the rule in action, Google: youtube "divisibility by 7" "large number"
It is entirely performed through mental calculation! Changing what must be changed the rule works for divisibility by 11 and 13.

dabral · Answer

shoonya I will second what Zoelef said, the rule for divisibility by 7 is irrelevant for the GMAT. The rule for 8 is important, although so far I haven't seen it being tested on the GMAT. I have seen the rule for 4 and 6 being tested on the actual GMAT. I have attached an image that gives you an example of an official GMAT problem where knowing the divisibility rule is a must.

Show SpoilerAnswer

A

Cheers,
Dabral

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gmatproblem-divisibilty.png [ 8.19 KiB | Viewed 4181 times ]

A

Cheers,
Dabral

Cadaver · Answer

Test for divisibility by 7. Double the last digit and subtract it from the remaining leading truncated number. If the result is divisible by 7, then so was the original number. Apply this rule over and over again as necessary. Example: 826. Twice 6 is 12. So take 12 from the truncated 82. Now 82-12=70. This is divisible by 7, so 826 is divisible by 7 also.

silvelho · Answer

This is a test for divisibility by 7, not a rule. It works because a multiple of seven is subtracted repetitively from the tested number. Observe that in 826, 126 is subtracted from 826. 126 mod 7 is equivalent to 56. This is true for any final digit. It would be quick and easier to subtract 56 instead of 126 because the 5 may be deduced (all you must know is the seven times table), eliminating the need of multiplying by 2.
The application of a rule is quicker than performing division. Try to apply the test to N = 3,218,576,816. It will take approximately the double of the time used to perform division. If you try my rule, give it a chance!, it will take less than ten seconds!
Regarding divisibility by 8 consider abc the three final digits of N and calculate c' ≣ (4 .a + c) mod 8; eliminate "a" and you have c'd; if 8|c'd then 8|N.
Example: N 6954722; abc = 722; c' ≣ (4 . 7 + 2) mod 8 ≣ 6; c'd = 26; 8 does not divide 26 and 26 mod 8 ≣ 2 (remainder of N/8. I like this rule because it is quick and gives the remainder of the division.

dharam831 · Answer

are there any rules of divisibility by 7 and 8?

Divisibility by 8 - Check divisibility of last 3 digits( unit's, hundred's and thousand's) of any number if it divisible then that number will be divisible by 8.
eg - 18088/8 = 088/8 = 11 = divisible.

Divisibility by 7 - Suppose we want to check whether 18088 is divisible by 7 or not.
step 1- Double the unit digit, here unit digit is 8. so double will be 2*8 = 16
step 2- Number left = 1808. Now substract 16 from 1808 = 1808-16 = 1792
step 3- Divide 1792/7 and check if you can otherwise repeat step 1 and step 2.
step 4- 179-(2*2) = 175
step 5- 17-(2*5) = 17-10 = 7 which is divisible by 7

So, 18088 is divisible by 7.

are there any rules of divisibility by 7 and 8

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