How many such 5s are there in the following number sequence each of which is immediately preceded by 3 or 4 but not immediately followed by 8 or 9?
3 5 9 5 4 5 5 3 5 8 4 5 6 7 3 5 7 5 5 4 5 2 3 5 1 0
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 5
Look at every 5 with its two neighbours. It counts only if the number just before it is 3 or 4 and the number just after it is not 8 or 9.
Counted: 4 5 5, 4 5 6, 3 5 7, 4 5 2 and 3 5 1, that is, 5 of them
Left out because of the number after: 3 5 9 and 3 5 8
Left out because of the number before: the 5s that come after 9, 5, 7 and 5