Remainders Question from MGMAT

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Remainders Question from MGMAT

by RoseSignet » Thu Dec 04, 2008 12:32 pm
Seven integers, x1, x2, x3, x4, x5, x6, and x7, are picked at random from the set of all integers between 10 and 110, inclusive. If each of these integers is divided by 7 and the 7 remainders are all added together, what would be the sum of the 7 remainders?

(1) The range of the remainders is 6.

(2) The seven integers are consecutive.

I'm not sure where to begin with this question.
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by kv_ajay » Thu Dec 04, 2008 12:55 pm
A) The range of remainders is 6 - Not useful since numbers could be 20(rem 6), 21 (rem 0), and then rest of the numbers could be anything with no exact remainders.
B) Seven integers are consecutive - which means one of them is divided by 7 so for example (11 to 18) - Each one has unique remainder and it is between 0 to 6. So the sum of remainders is (1+2+3+4+5+6+0)

So B sufficient.

Let me know if this is correct.

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by RoseSignet » Thu Dec 04, 2008 1:06 pm
kv_ajay. Yeah you got it!

The remainder has to be less than the divisor (in this case 7) so that would mean that the other remainders would have to be between 1 and 6. The remainder for the number that is divisible by 7 would be zero.

Thanks that was very helpful :)
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Re: Remainders Question from MGMAT

by lunarpower » Fri Dec 05, 2008 3:16 am
RoseSignet wrote:I'm not sure where to begin with this question.
more to the point, here's a helpful TECHNIQUE for beginning problems like this one.

if you have a problem about REMAINDERS, you should view that problem as an opportunity for PATTERN RECOGNITION.
there are lots of topics that lead to recognition of common patterns - i.e., remainders aren't the only topic of such problems - but, in remainder problems, CLEAR patterns tend to emerge QUICKLY if you start testing numbers in some sort of systematic manner.

in this problem, therefore, and in problems like it:
if you don't immediately see a better technique, you should JUST START PLUGGING IN SAMPLE NUMBERS AND LOOK FOR A PATTERN.

let's do this with the first several of "the integers between 10 and 110, inclusive" (the set in question), and see what happens:
10 --> remainder = 3
11 --> 4
12 --> 5
13 --> 6
14 --> 0
15 --> 1
16 --> 2
17 --> 3
18 --> 4
19 --> 5
20 --> 6
21 --> 0
by this point it's pretty clear what's happening: the remainders go 0, 1, 2, 3, 4, 5, 6, and then repeat the same sequence over and over again.

the posters above have the correct answer nailed down already, but note how much easier it is to resolve (b) once you've found this pattern.
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