Bars of Soap

When 2007 bars of soap are packed into N N boxes, where N N is a positive integer, there is a remainder of 5. How many possible values of N N are there?


The answer is 14.

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2 solutions

Zekiah Villamayor
Nov 21, 2016

Relevant wiki: Number of Factors

From the first sentence, we know that when 2007 is divided by N N , the remainder is 5.

Or equivalently, 2007 05 ( m o d N ) 2007 \equiv 0 5 \pmod N , or 2007 5 = 2002 0 ( m o d N ) 2007 - 5 = 2002 \equiv 0 \pmod N .

So 2002 must be divisible by N N , with and additional constraint that N N is larger than 5.

And we want to find the total number of possible values of N N , which is simply the number of positive divisors of 2002 larger than 5.

2002 factors to 2 1 × 7 1 × 1 1 1 × 1 3 1 2^1\times7^1\times11^1\times13^1 . So total number of factors of 2002 is ( 1 + 1 ) ( 1 + 1 ) ( 1 + 1 ) ( 1 + 1 ) = 16 (1+1)(1+1)(1+1)(1+1) = 16 .

But the only positive divisors of 2002 less than or equal to 5 are (1,2), so we need to discount two of these divisors.

Hence, there are 16 2 = 14 16-2 = \boxed{14} possible choices of N N .

Your question doesn't say the sizes of each box must be equal. Could you add this detail in your question, so it would be easier to understand and wouldn't be misunderstood.

Tarmo Taipale - 4 years, 6 months ago

Why N is larger than 5?

Abdelhamid Saadi - 4 years, 6 months ago

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If N was lesser than 5, the remainder could not be 5. For example, if the size of the box is 2, the remainder can't be 5, as 4 of the remaining 5 soap bars could be packed in boxes of 2. Instead, the remainder would be 1.

Tarmo Taipale - 4 years, 6 months ago

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We can have 1001 boxes of 2 and remainder of 5.

Abdelhamid Saadi - 4 years, 6 months ago

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@Abdelhamid Saadi Well, the question itself is seemingly not clear, it doesn't say as many bars as possible should be packed, but it seems that we're supposed to think so.

Tarmo Taipale - 4 years, 6 months ago

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@Tarmo Taipale We do assume that each N of box, contain the same number of bars.

Niranjan Khanderia - 4 years, 6 months ago

@Abdelhamid Saadi N can be 2. But there can not be 1001 boxes. In that case there will be only 2 in a box and can not have 5 remainder. Similarly can not be 1001 boxes for the same reason. So these two are not possible.

Niranjan Khanderia - 4 years, 6 months ago

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@Niranjan Khanderia You may say that boxes are supplied as needed

Abdelhamid Saadi - 4 years, 6 months ago

@Abdelhamid Saadi No. We have to fill 1003 boxes with two bares and the remainder is 1. This is not stated but implied. Adding "For one value of N, box are of same size, and all boxes are filled full till left bars can not fill the next box full." .or similar statement can make the problem clear. Still it is reasonable to expect this as implied.

Niranjan Khanderia - 4 years, 6 months ago

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@Niranjan Khanderia That's what I thought too. But don't you think it would be more clear if it's mentioned that as many boxes as possible must be filled and all the boxes are same size? With that, misunderstandings could be avoided.

Tarmo Taipale - 4 years, 6 months ago

It is not the boxes. We can not have one or two bars per box, that is 2002 or 1001 boxes.

Niranjan Khanderia - 4 years, 6 months ago

2007-5=2002. What ever factors we get is the number N. We may use number theory or simple logic as under.
Factors of 2002 are 2, 7, 11, 13.
Multiplying one at a time we have 4 box arrangments........Bars=..2..7..11..13.....................N=....1001.....286....182....154
Multiplying two at a time we have 6 box arrangments........Bars=..14..22..26..77..91..143 ....N=..143..91..77...26..22..14
Multiplying three at a time we have 4 box arrangments. But with 7 11 13=1001 boxes there will be only two in a box and we can not have 5 remainder. Hence we get only 3 box arrangments.............Bars=..154..182..286............... ...N=...13...11...7
Multiplying four at a time we have 1 box arrangment.........Bars=...2002...............................N=..1..
Total 4+6+3+1=14.



C(4,1)= 4 .......C(4,2)= 6 ......C(4,3)=4...minus one as explained above= 3 .......C(4,4)= 1 .

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