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Question

In the following finite sequence of integers, how many 9s are divisible by the integers immediately preceding them?

8, 3, 4, 9, 3, 5, 9, 5, 9, 9, 9, 4, 5, 9, 5, 6, 3, 3, 5, 7, 2, 3, 9, 9, 8, 9, 3, 9, 1, 9, 4

The correct answer is

6

Sequence Analysis and Divisibility

The problem asks us to examine a given finite sequence of integers and count how many times the integer '9' is directly preceded by an integer that divides '9' without leaving a remainder.

The given sequence is:

8, 3, 4, 9, 3, 5, 9, 5, 9, 9, 9, 4, 5, 9, 5, 6, 3, 3, 5, 7, 2, 3, 9, 9, 8, 9, 3, 9, 1, 9, 4

We need to find every instance of '9' in the sequence and look at the integer immediately before it. Then, we check if '9' is divisible by that preceding integer. A number 'a' is divisible by a number 'b' if the division $a \div b$ results in an integer.

Let's go through the sequence and identify all occurrences of '9' and their preceding numbers:

  • The first 9 is preceded by 4. Is 9 divisible by 4? $9 \div 4 = 2.25$. No.
  • The second 9 is preceded by 5. Is 9 divisible by 5? $9 \div 5 = 1.8$. No.
  • The third 9 is preceded by 5. Is 9 divisible by 5? $9 \div 5 = 1.8$. No.
  • The fourth 9 is preceded by 9. Is 9 divisible by 9? $9 \div 9 = 1$. Yes. (Count 1)
  • The fifth 9 is preceded by 9. Is 9 divisible by 9? $9 \div 9 = 1$. Yes. (Count 2)
  • The sixth 9 is preceded by 9. Is 9 divisible by 9? $9 \div 9 = 1$. Yes. (Count 3)
  • The seventh 9 is preceded by 5. Is 9 divisible by 5? $9 \div 5 = 1.8$. No.
  • The eighth 9 is preceded by 3. Is 9 divisible by 3? $9 \div 3 = 3$. Yes. (Count 4)
  • The ninth 9 is preceded by 9. Is 9 divisible by 9? $9 \div 9 = 1$. Yes. (Count 5)
  • The tenth 9 is preceded by 8. Is 9 divisible by 8? $9 \div 8 = 1.125$. No.
  • The eleventh 9 is preceded by 3. Is 9 divisible by 3? $9 \div 3 = 3$. Yes. (Count 6)
  • The twelfth 9 is preceded by 1. Is 9 divisible by 1? $9 \div 1 = 9$. Yes. (Count 7) -- Let's re-check the count based on the sequence carefully.

Let's list the pairs of (preceding number, 9) as they appear in the sequence and check for divisibility:

Index of 9 Preceding Number Divisibility Check ($9 \div \text{Preceding}$) Is 9 Divisible?
4 4 $9 \div 4 = 2.25$ No
7 5 $9 \div 5 = 1.8$ No
9 5 $9 \div 5 = 1.8$ No
10 9 $9 \div 9 = 1$ Yes
11 9 $9 \div 9 = 1$ Yes
12 9 $9 \div 9 = 1$ Yes
14 5 $9 \div 5 = 1.8$ No
23 3 $9 \div 3 = 3$ Yes
24 9 $9 \div 9 = 1$ Yes
26 8 $9 \div 8 = 1.125$ No
28 3 $9 \div 3 = 3$ Yes
30 1 $9 \div 1 = 9$ Yes

Counting the cases where 9 is divisible by the preceding number, we find there are 7 such occurrences based on the table above.

Let's carefully re-read the sequence and count: 8, 3, 4, 9 (prec. 4 - No), 3, 5, 9 (prec. 5 - No), 5, 9 (prec. 5 - No), 9 (prec. 9 - Yes), 9 (prec. 9 - Yes), 4, 5, 9 (prec. 5 - No), 5, 6, 3, 3, 5, 7, 2, 3, 9 (prec. 3 - Yes), 9 (prec. 9 - Yes), 8, 9 (prec. 8 - No), 3, 9 (prec. 3 - Yes), 1, 9 (prec. 1 - Yes), 4.

Let's list the pairs (preceding number, 9) again and count only the 'Yes' cases:

  • (4, 9) - No
  • (5, 9) - No
  • (5, 9) - No
  • (9, 9) - Yes (Count 1)
  • (9, 9) - Yes (Count 2)
  • (9, 9) - Yes (Count 3)
  • (5, 9) - No
  • (3, 9) - Yes (Count 4)
  • (9, 9) - Yes (Count 5)
  • (8, 9) - No
  • (3, 9) - Yes (Count 6)
  • (1, 9) - Yes (Count 7)

Revisiting the sequence carefully and checking positions of 9 and its preceding number:

Sequence: 8, 3, 4, 9, 3, 5, 9, 5, 9, 9, 9, 4, 5, 9, 5, 6, 3, 3, 5, 7, 2, 3, 9, 9, 8, 9, 3, 9, 1, 9, 4

Pairs (Preceding, 9) where 9 appears:

  1. (4, 9): $9 \div 4$ No
  2. (5, 9): $9 \div 5$ No
  3. (5, 9): $9 \div 5$ No
  4. (9, 9): $9 \div 9$ Yes (1)
  5. (9, 9): $9 \div 9$ Yes (2)
  6. (9, 9): $9 \div 9$ Yes (3)
  7. (5, 9): $9 \div 5$ No
  8. (3, 9): $9 \div 3$ Yes (4)
  9. (9, 9): $9 \div 9$ Yes (5)
  10. (8, 9): $9 \div 8$ No
  11. (3, 9): $9 \div 3$ Yes (6)
  12. (1, 9): $9 \div 1$ Yes (7)

There seems to be a discrepancy with the provided answer option suggesting 6. Let's review the sequence one final time to ensure all 9s are found and checked with their correct preceding number:

8, 3, 4, 9 (prec. 4, No)
3, 5, 9 (prec. 5, No)
5, 9 (prec. 5, No)
9 (prec. 9, Yes) - Count 1
9 (prec. 9, Yes) - Count 2
4, 5, 9 (prec. 5, No)
5, 6, 3, 3, 5, 7, 2, 3, 9 (prec. 3, Yes) - Count 3
9 (prec. 9, Yes) - Count 4
8, 9 (prec. 8, No)
3, 9 (prec. 3, Yes) - Count 5
1, 9 (prec. 1, Yes) - Count 6
4

Listing the pairs (preceding number, 9) from the sequence again:

(4, 9) - No
(5, 9) - No
(5, 9) - No
(9, 9) - Yes (1)
(9, 9) - Yes (2)
(9, 9) - Yes (3)
(5, 9) - No
(3, 9) - Yes (4)
(9, 9) - Yes (5)
(8, 9) - No
(3, 9) - Yes (6)
(1, 9) - Yes (7)

My count remains 7. However, the correct answer option provided is 6. Let me check the sequence one more time to ensure I haven't missed any 9s or misidentified a preceding number.

Sequence: 8, 3, 4, 9, 3, 5, 9, 5, 9, 9, 9, 4, 5, 9, 5, 6, 3, 3, 5, 7, 2, 3, 9, 9, 8, 9, 3, 9, 1, 9, 4

Pairs (Preceding, 9) for every 9 in the sequence:

  • Index 4: (4, 9) - No
  • Index 7: (5, 9) - No
  • Index 9: (5, 9) - No
  • Index 10: (9, 9) - Yes (1)
  • Index 11: (9, 9) - Yes (2)
  • Index 14: (5, 9) - No
  • Index 23: (3, 9) - Yes (3)
  • Index 24: (9, 9) - Yes (4)
  • Index 26: (8, 9) - No
  • Index 28: (3, 9) - Yes (5)
  • Index 30: (1, 9) - Yes (6)

Okay, I see the mistake in my previous manual counts. There are only 11 occurrences of '9' that have a preceding number in the sequence (the first digit 8 does not have a preceding number). The pairs are derived from the indices of the 9s and the number at the index immediately before it.

Let's list the indices of the 9s and their preceding numbers' values again:

Position of 9 Preceding Number (at Position - 1) Divisibility Check ($9 \div \text{Preceding}$) Is 9 Divisible?
4 4 $9 \div 4 = 2.25$ No
7 5 $9 \div 5 = 1.8$ No
9 5 $9 \div 5 = 1.8$ No
10 9 $9 \div 9 = 1$ Yes
11 9 $9 \div 9 = 1$ Yes
14 5 $9 \div 5 = 1.8$ No
23 3 $9 \div 3 = 3$ Yes
24 9 $9 \div 9 = 1$ Yes
26 8 $9 \div 8 = 1.125$ No
28 3 $9 \div 3 = 3$ Yes
30 1 $9 \div 1 = 9$ Yes

Counting the 'Yes' entries in the table:

  • (9, 9) - Yes (from index 10)
  • (9, 9) - Yes (from index 11)
  • (3, 9) - Yes (from index 23)
  • (9, 9) - Yes (from index 24)
  • (3, 9) - Yes (from index 28)
  • (1, 9) - Yes (from index 30)

There are exactly 6 instances where a '9' is divisible by the integer immediately preceding it.

The preceding integers that divide 9 are 9, 3, and 1.

  • 9 is divisible by 9.
  • 9 is divisible by 3.
  • 9 is divisible by 1.

We counted these occurrences in the sequence: (9,9), (9,9), (9,9), (3,9), (9,9), (3,9), (1,9). Oh wait, this list includes the pair (5,9) before the consecutive 9s. Let's be systematic.

Sequence: 8, 3, 4, 9, 3, 5, 9, 5, 9, 9, 9, 4, 5, 9, 5, 6, 3, 3, 5, 7, 2, 3, 9, 9, 8, 9, 3, 9, 1, 9, 4

Let's mark the 9s that are divisible by their preceding number:

8, 3, 4, 9(No), 3, 5, 9(No), 5, 9(No), 9(Yes, by 9), 9(Yes, by 9), 4, 5, 9(No), 5, 6, 3, 3, 5, 7, 2, 3, 9(Yes, by 3), 9(Yes, by 9), 8, 9(No), 3, 9(Yes, by 3), 1, 9(Yes, by 1), 4

Counting the 'Yes' cases: 1, 2, 3, 4, 5, 6. There are 6 such occurrences.

The final answer is $\boxed{6}$.

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