A New Method to divide Numbers by 8 (Eight)!

Consider the following procedure for dividing the three-digit number 375 375 by 8 8 :

  • Write down the number formed by the first two digits, namely 37 37 .

  • Multiply this by 2 2 to get 74 74 .

  • Add to this, the units digit of 375 375 (the original number), obtaining 74 + 5 = 79 74+5=79 .

  • Then divide it by 8 8 to get 9 9 with a remainder of 7 7 .

  • Add this result ( 9 9 , remainder 7 7 ) with 37 37 (the first two digits of the original number) to get your answer: 46 46 , remainder 7 7 .

Thus 375 375 divided by 8 8 equals 46 46 with a remainder of 7 7 .

Does this method always work for three-digit numbers? Why, or why not?

Yes, this method always works for three-digit numbers. No, this method doesn't always work for three-digit numbers.

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2 solutions

Mehul Arora
Sep 15, 2015

Basic Knowledge: Any Three digit number will be in the form of 100 x + 10 y + z 100x+10y+z where x is it's hundred's digit, y is the tens digit and z is the units digit. E.g. 341= 100 × 3 + 10 × 4 + 1 100 \times 3+ 10 \times 4+ 1

x y z \overline {xyz} represents the number as a value. If we write that the number = x y z xyz it might be percieved for x × y × z x \times y \times z

Let the number be here x y z = 100 x + 10 y + z \overline {xyz} = 100x+10y+z

According to the procedure, The number formed by the first two digits = x y = 10 x + y \overline {xy}=10x+y

Multiply by two to get 20 x + 2 y 20x+2y

Adding the units digit, we get 20 x + 2 y + z 20x+2y+z

Divide it by Eight. 20 x + 2 y + z 8 \dfrac {20x+2y+z}{8}

Let us ignore the step where we get the remainder and the quotient and we add them to the original two digit number, because it makes no difference whether we divide and add or we add first and divide. (E.g. We break a number 341 into 100+241. We divide them individually by 8 getting remainders 4 and 1. We add them, and we get 5 as the remainder which is the remainder when 341 is divided by 8).

Instead, we directly add our number i.e. 20 x + 2 y + z 8 \dfrac {20x+2y+z}{8} to our original two digit number.

Adding, 80 x + 8 y + 20 x + 2 y + z 8 \dfrac {80x+8y+20x+2y+z}{8}

100 x + 10 y + z 8 \Rightarrow \dfrac {100x+10y+z}{8}

x y z 8 \Rightarrow \dfrac {\overline {xyz}}{8} Thus this would give us the remainder and the quotient when the original there digit number is divided by 8

Woot! Thus we get the desired result and the method works!. :)

Moderator note:

While I understand what you are attempting to say, it is not expressed clearly. I am concerned that someone who doesn't already know how to solve this problem, would not be unable to understand what you are saying.

Lovely Problem! @Satyajit Mohanty

Mehul Arora - 5 years, 9 months ago

Lovely Solution! @Mehul Arora

Nihar Mahajan - 5 years, 9 months ago

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Thanks @Nihar Mahajan :D :D

Mehul Arora - 5 years, 9 months ago

While I understand what you are attempting to say, it is not expressed clearly. I am concerned that someone who doesn't already know how to solve this problem, would not be unable to understand what you are saying.

Calvin Lin Staff - 5 years, 9 months ago

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@Calvin Lin Which part of my solution should I improve?

Mehul Arora - 5 years, 9 months ago

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Overall, make it much clearer why/how "the remainder when xyz is divided by 8 equals the remainder when (20x+2y+z) is divided by 8. While I see the steps involved, that explanation of the steps is not presented.

  1. Make it clear what you are doing, and why it's valid. What you have written is hard to follow and understand, especially for someone who doesn't know how why it's true. Just imagine if you showed this solution to your younger brother/sister. Would they understand it without any further prompting from you?
  2. What do you mean by "Let us ignore the step where we get the remainder and the quotient". You are asked the find the remainder, so why ignore it.
  3. How does "Instead, we directly add our number" prove the statement? If we wanted to show that " 5 = 4 + 1 ", are we allowed to add stuff to one side without adding to another side?

Calvin Lin Staff - 5 years, 9 months ago

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@Calvin Lin @Calvin Lin I cleaned up my solution a bit. Is it better now? If anything needs to be changed, I'll do that Tomorrow morning :)

Additional info:- I have a younger brother but he's only 9 years. So pretty much, he won't be able to understand all of this stuff :P

Mehul Arora - 5 years, 9 months ago

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@Mehul Arora Haha, well not a younger brother than, but a friend who hasn't solved this problem.

It still doesn't address the concerns that I brought up above. You have not explained why "the remainder when xyz is divided by 8 equals the remainder when (20x+2y+z) is divided by 8", which is the point of the solution.

You have all of the steps needed, but you do not have the phrasing / logical connections between these statements.

Calvin Lin Staff - 5 years, 9 months ago
Arjen Vreugdenhil
Sep 20, 2015

Let the number be a b c \overline{abc} . The successive steps have us calculate: a b ; \overline{ab}; 2 a b ; 2\overline{ab}; 2 a b + c ; 2\overline{ab}+c; 2 a b + c 8 ; \frac{2\overline{ab}+c}8; a b + 2 a b + c 8 = 10 a b + c 8 = a b c 8 . \overline{ab} + \frac{2\overline{ab}+c}8 = \frac{10\overline{ab}+c}8 = \frac{\overline{abc}}8.

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