Discussion

In this number series

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

how many 5 are there such that their left number and right number both are odd ?

Answer: B

There are 4 such 5s.

Check each 5 in 1 3 5 5 4 5 5 7 3 3 5 5 5 2 8 7 2 5 3 and look at its two neighbours:

3rd digit (3, 5, 5): both odd, counted.
4th digit (5, 5, 4): right is even.
6th digit (4, 5, 5): left is even.
7th digit (5, 5, 7): both odd, counted.
11th digit (3, 5, 5): both odd, counted.
12th digit (5, 5, 5): both odd, counted.
13th digit (5, 5, 2): right is even.
18th digit (2, 5, 3): left is even.

That gives 4. The answer 8 is simply the total number of 5s in the series, the trap for anyone who ignores the condition.

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