Given that T1 = 23
and Tn = T(n-1) - 3
To Find:
For what n
Tn = -4
T1 = 23
T2 = 23-3
T3 = 23-3-3 or 23 - 2*(3)
T4 = 23 - 3*(3)
From the above sequence we know that
Tn = T1 - (n-1)*3 = -4
=> 23- (n-1)*3 = -4
n-1 = 9
N = 10
Hence Answer is C
Hope it helps you
Regards,
Uva.
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Can you explain this problem?
Source: Beat The GMAT — Problem Solving |
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Hi yumi2012,
Uva@90 has provided a solid explanation for the "math" behind this question. The most important part is to understand that the sequence is decreasing by 3 with every term. We're given the value of the first term (23) and we're asked to figure out which term = -4.
If you can't see the "algebra" behind this question, then you could just "brute force" the sequence. Here's how:
1st = 23
2nd = 20
3rd = 17
4th = 14
5th = 11
6th = 8
7th = 5
8th = 2
9th = -1
10th = -4
Final Answer: C
GMAT assassins aren't born, they're made,
Rich
Uva@90 has provided a solid explanation for the "math" behind this question. The most important part is to understand that the sequence is decreasing by 3 with every term. We're given the value of the first term (23) and we're asked to figure out which term = -4.
If you can't see the "algebra" behind this question, then you could just "brute force" the sequence. Here's how:
1st = 23
2nd = 20
3rd = 17
4th = 14
5th = 11
6th = 8
7th = 5
8th = 2
9th = -1
10th = -4
Final Answer: C
GMAT assassins aren't born, they're made,
Rich
-
GMATGuruNY
- GMAT Instructor
- Posts: 15539
- Location: New York, NY
- Thanked: 13060 times
- GMAT Score:790
Alternate approach:
T(n) = T(n) - 3.
According to the formula above, each term is 3 less than the preceding term.
The result is an EVENLY SPACED SET.
To count the number of integers in an evenly spaced set, use the following formula:
(biggest - smallest)/interval + 1.
The interval is the distance between successive terms.
Here, the interval is 3, since each term is 3 less than the preceding term.
Since the biggest term = 23 and the smallest term = -4, we get:
Number of terms = (biggest - smallest)/interval + 1 = [23 - (-4)]/3 + 1 = 10.
The correct answer is C.
T(n) = T(n) - 3.
According to the formula above, each term is 3 less than the preceding term.
The result is an EVENLY SPACED SET.
To count the number of integers in an evenly spaced set, use the following formula:
(biggest - smallest)/interval + 1.
The interval is the distance between successive terms.
Here, the interval is 3, since each term is 3 less than the preceding term.
Since the biggest term = 23 and the smallest term = -4, we get:
Number of terms = (biggest - smallest)/interval + 1 = [23 - (-4)]/3 + 1 = 10.
The correct answer is C.
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Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.
As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.
For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
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