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805+ (Hard)|   Sequences|      
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gracie
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The mean of an ascending sequence of positive consecutive two-digit in Updated on: 31 Dec 2018, 21:52
Originally posted by gracie on 30 Dec 2018, 23:12.
Last edited by gracie on 31 Dec 2018, 21:52, edited 1 time in total.
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Difficulty:

95% (hard)

Question Stats:

45% (02:59) correct 55% (02:47) wrong based on 55 sessions

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The mean of an ascending sequence of positive consecutive two-digit integers is the reverse of the first term. If the sum of the first and last terms is less than 100, how many such sequences are there?

A. 4
B. 5
C. 6
D. 7
E. 8
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C
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chetan2u
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The mean of an ascending sequence of positive consecutive two-digit in 31 Dec 2018, 20:59
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gracie
The mean of an ascending sequence of positive consecutive two-digit integers is the reverse of the first term. If the sum of the first and last terms is less than 100, how many such sequences are there?

A. 1
B. 2
C. 3
D. 4
E. 5
Show more

Since the terms are consecutive, sum of first and last term will be equal to 2 times the mean

Let the sequence start with 12, then 12....21......30.. SUM =2*21=42
Starting with 13, then 13....31...., Sum =2*31=62
Starting with 14, then 14...41..., Sum = 2*41=84
Mean of 51 will make the sum 102.. not possible

Now let the sequence start with 23...sum =2*32=64
Starting with 24..42.., sum=2*42=84
Mean of 52 will mean greater than 100

Now let the sequence start in 30s
So 34...43... 2*43=86 possible

No other possible as starting with 45 will lead to to a mean of 54..

Total sequences possible are starting with 12, 13, 14, 23, 24, and 34..

Please relook into what was intended to ask.
What is the source??
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The mean of an ascending sequence of positive consecutive two-digit in 31 Dec 2018, 21:51
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chetan2u
gracie
The mean of an ascending sequence of positive consecutive two-digit integers is the reverse of the first term. If the sum of the first and last terms is less than 100, how many such sequences are there?

A. 1
B. 2
C. 3
D. 4
E. 5

Since the terms are consecutive, sum of first and last term will be equal to 2 times the mean

Let the sequence start with 12, then 12....21......30.. SUM =2*21=42
Starting with 13, then 13....31...., Sum =2*31=62
Starting with 14, then 14...41..., Sum = 2*41=84
Mean of 51 will make the sum 102.. not possible

Now let the sequence start with 23...sum =2*32=64
Starting with 24..42.., sum=2*42=84
Mean of 52 will mean greater than 100

Now let the sequence start in 30s
So 34...43... 2*43=86 possible

No other possible as starting with 45 will lead to to a mean of 54..

Total sequences possible are starting with 12, 13, 14, 23, 24, and 34..

Please relook into what was intended to ask.
What is the source??
Show more

hi chetan2u,
thank you so much for pointing out my mistake.
I was erroneously assuming that, because 2*51 exceeded 100, 14 would be the greatest possible first term for such a
sequence, and did not check for further possibilities, as I clearly should have.
I apologize for this and have edited the answer choices appropriately.
gracie
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The mean of an ascending sequence of positive consecutive two-digit in 30 Mar 2026, 02:28
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Quite tricky but the idea is that a+l/2 < 50 which means that avg of the consec numbers is going to be under 50 for sure.

Now the second digit must be smaller than 1st digit if we take avg so that the when reverse is taken smallest number is smaller than avg.

Combining these two facts,
we have 43,42,41 as options under 40s.
We have 32,31 under 30s.
We have 21 under 20s.

Therefore 6 values.
Also to note that 40 or 30 or 20 is not valid because reversing will make first number as a single digit number.

___________________________

Bunuel how often is Sequences and Series asked in GMAT Focus edition as I barely see any sequence related questions in GMAT Club tests. Thankyou!

Answer: Option C
gracie
The mean of an ascending sequence of positive consecutive two-digit integers is the reverse of the first term. If the sum of the first and last terms is less than 100, how many such sequences are there?

A. 4
B. 5
C. 6
D. 7
E. 8
Show more
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