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The sum of four consecutive odd numbers is equal to the sum [#permalink]
21 Sep 2011, 10:52

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The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed?

The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed? (A) 12 (B) 17 (C) 25 (D) 33 (E) 50

Four consecutive odd numbers: k-2, k, k+2, k+4 Three consecutive even numbers: n-2, n, n+2

Yeah!!! So I see. I'll wait for few others to point out the error. I can't think where I may be going wrong. BTW, what's the source of the problem?
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The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed? (A) 12 (B) 17 (C) 25 (D) 33 (E) 50

Four consecutive odd numbers: k-2, k, k+2, k+4 Three consecutive even numbers: n-2, n, n+2

All n's that's divisible by 4 will have an integral k. So, we need to find out how many such n's are available within given range:

We know, 101<n<200 104<=n<=196

Count=(196-104)/4+1=92/4+1=23+1=24

Ans: 24.

k-2, k, k+2, k+4 n-2, n, n+2

Why did you take the minus??? Why not : Four consecutive odd numbers: O, O+2, O+4, O+6 Three consecutive even numbers: E, E+2, E+4 O+O+2+ O+4+ O+6 = E+ E+2+E+4 4O + 12 = 3E + 6 4O + 6 = 3E 4O = 3(E-2) O = 3(E-2)/4

1+3+5+7 = 16 3+5+7+9 = 24 5+7+9+11=32 ... so you can see the sum of 4 odd numbers increment by 8.

Therefore all even number that are divisible by 8 between 101-200 will have a possible series that adds up to it.. therefore 200-101/8 gives you 12.75 = 12!

The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed? (A) 12 (B) 17 (C) 25 (D) 33 (E) 50

Four consecutive odd numbers: k-2, k, k+2, k+4 Three consecutive even numbers: n-2, n, n+2

Why did I consider: n-2, n, n+2: For the sake of simplicity, because we are given that 101<n<200 (Middle term of the even number)

According to given condition: Sum of 4 consecutive integers=Sum of 3 even integers k-2+k+k+2+k+4=n-2+n+n+2 4k+4=3n 4(k+1)=3n k=(3/4)n-1;

Now, we know that n(middle term of the even sequence) is between 101 and 200, exclusive

So, 101<n<200

But, we should also conform to the fact that k is ODD.

How can we get k as odd Say n=102; k=(3/4)*102-1=76.5-1=75.5(It is NOT ODD); so n=102 IS not a possible/valid sequence Likewise n=103; will also not give k as odd; n=104; k=(3/4)*104-1=77(ODD)

We see that if n=A multiple of 8, then k becomes ODD. How so?

4*Even=4*2x=Even So, n must be in the form of 8x.

105,106,107,108,109,110,111(They are not divisible by 8), thus k can't be an ODD integer

112 is divisible by 8.

So, if we find all the values from 101 to 200 that are divisible by 8, we will have our count. The sequence will be.

1st: 102,104,106 2nd: 110,112,114 3rd: 118,120,122 ... 12th: 190,192,194 Note: we just have to care about the middle term. The first term and last term will be follow: +-2.

Also, so far k is ANY ODD integer, we are good.
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The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed? (A) 12 (B) 17 (C) 25 (D) 33 (E) 50

Sum of four consecutive odd numbers: (2a - 3) + (2a - 1) + (2a + 1) + (2a + 3) = 8a

Sum of three consecutive even numbers: (2b - 2) + 2b + (2b + 2) = 6b

Given 8a = 6b or a/b = 3/4, a and b can be any integers. So, 'a' has to be a multiple of 3 and 'b' has to be a multiple of 4. Possible solutions are: a = 3, b = 4; a = 6, b = 8; a = 9, b = 12 etc Since 101 < 2b < 200 i.e. 51 <= b < 100 Since b also has to be a multiple of 4, the values that b can take are 52, 56, 60, 64 ... 96 Number of values b can take = (Last term - First term)/Common Difference + 1 = (96 - 52)/4 + 1 = 12
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Re: The sum of four consecutive odd numbers is equal to the sum [#permalink]
13 Nov 2013, 01:48

bindiyajoisher wrote:

The sum of four consecutive odd numbers is equal to the sum of 3 consecutive even numbers. Given that the middle term of the even numbers is greater than 101 and lesser than 200, how many such sequences can be formed?

(A) 12 (B) 17 (C) 25 (D) 33 (E) 50

We need to choose n : 101 < n < 200 and n is even, and (n-2) + n + (n+2) = sum of 4 consecutive odd numbers.

I am assuming the 4 consecutive odd numbers to be e-3, e-1, e+1, e+3 where e could be an even integer > 3.

Hence: (n-2) + n + (n+2) = e-3 + e-1 + e+1 + e+3

3n = 4e

Let us analyse the above equation. For the above equation to hold true, e should be a multiple of 6 (as it should have a 3 in it and is an even) and n should be a multiple of 8 (because n should have factors 4 and 2 as 3 is a prime already and 4e has these factors in RHS). This is the least requirement.

So every multiple of 8 will satisfy the above equation if I do not restrict e.

So possible values of n(for no restriction on e) = all multiples of 8 between 101 and 200 = 12.

Hope I made my point clear

gmatclubot

Re: The sum of four consecutive odd numbers is equal to the sum
[#permalink]
13 Nov 2013, 01:48