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Bunuel
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You're right. I have edited my explanation and the correct answer should be B. Sorry for the oversight and thanks for pointing it out.
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Bunuel
If the sum of the 6th, 11th and 30th terms of an arithmetic progression is equal to the sum of the 3rd, 8th, 18th and the 25th terms, then the sum of the first how many terms of the series is zero?

A. 15
B. 13
C. 12
D. 11
E. 7


OA is B

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Bunuel
If the sum of the 6th, 11th and 30th terms of an arithmetic progression is equal to the sum of the 3rd, 8th, 18th and the 25th terms, then the sum of the first how many terms of the series is zero?

A. 15
B. 13
C. 12
D. 11
E. 7


Given:
a + 5d + a + 10d + a + 29d = a + 2d + a + 7d + a + 17d + a + 24d
a + 6d = 0

This gives us that the 7th term is 0. This means that the first 6 terms are negative and at the same fixed interval. The 6 terms after the 7th term will be positive but also at the same fixed interval (since difference between 2 consecutive terms in an AP is constant).

Hence the AP will look something like this (an example): -12, -10, -8, -6, -4, -2, 0, 2, 4, 6, 8, 10, 12, 14, 16, ....
Hence sum of first 6 + 1 + 6 = 13 terms will be 0.

Answer (B)
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Take a second to think

Use 1 as the arithmetic progression

Use the answers:
If terms 1 - 12 sum to 0, then the 13th term starts at 1

13th = 1, 14th = 2, ....

Now knowing this, calculate how much each term would be worth at its respective starting point

From here, we can start at the 14th term = 1

Meaning 6th, 11th, 3rd and 8th are irrelevant

18th term = 5, 25th term = 12, 30th term = 17

Making both sides =, 5 + 12 = 17

Answer: B :)
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