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Re: Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
chetan2u CAN YOU EXPLAIN THE CRUX "You remove a1, and you get the average of remaining 51 numbers down by 1.
Thus this number has to be 51+1(itself) less than the average."
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Re: Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
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Bunuel wrote:
Let \(a_1\), \(a_2\), ... , \(a_{52}\) be positive integers such that \(a_1 < a_2 < ... < a_{52}\). Suppose, their arithmetic mean is one less than the arithmetic mean of \(a_2\), \(a_3\), ..., \(a_{52}\). If \(a_{52} = 100\), then the largest possible value of \(a_1\) is

A. 20
B. 23
C. 45
D. 48
E. 50

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Let S = a2 + a3 + … + a51 + 100, i.e., the sum of the 52 terms except the first term. We can create the equation:

(a1 + S)/52 + 1 = S/51

(a1 + S)/52 + 52/52 = S/51

(a1 + S + 52)/52 = S/51

51(a1 + S + 52) = 52S

51a1 + 51S + 2652 = 52S

51a1 = S - 2652

a1 = S/51 - 52

The largest value of S is a2 + a3 + … + a51 + 100 = 50 + 51 + … + 99 + 100 = (50 + 100)/2 x 51 = 75 x 51 = 75 x 51. So, if S = 75 x 51, then a1 = S/51 - 52 = 75 - 52 = 23 would be the largest possible value.

Answer: B
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Re: Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
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varun325 wrote:
chetan2u CAN YOU EXPLAIN THE CRUX "You remove a1, and you get the average of remaining 51 numbers down by 1.
Thus this number has to be 51+1(itself) less than the average."



Look say average of 51 numbers is a, so sum is 51*a. But say average of 52 numbers is a-1 now, that is 52(a-1)=52a-52.
As you can see if 52nd number was a, the average would remain as a and sum as 52a.

But the new average is 52 less than 52a, that is it is same as removing 1 from each of the 52 number.
This is basically visualising the question and at many times this can make a question very simple and avoid complex calculations.
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Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
For a1 can be max, a2 until a51 must be maximum too
So a2 will be 100 - 50 = 50
And the mean will be (100 + 50)/2 = 75

If we just continue the sequence by giving a1 = 49, the mean will be (100 + 49)/2 = 74.5
If we want to get rid this 0.5, simply deduct 49 by (0.5*52)
= 49 - 26
= 23
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Re: Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
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Re: Let a1, a2, ... , a52 be positive integers such that a1 < a2 < ... < a [#permalink]
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