How many 4’s are there in the following number series, which are preceded by 2…
2016
How many 4’s are there in the following number series, which are preceded by 2 but followed by 3? Number Series: 1 3 2 4 3 4 5 6 7 4 2 1 6 2 4 5 3 1 2 4 3 6 1 4 7 2
Answer: B. Two — ConceptIn a “preceded by / followed by” series-counting question, a term is counted only when BOTH of its immediate neighbours match the stated conditions at…
- A.
One
- B.
Two
- C.
Three
- D.
Four
Attempted by 292 students.
Show answer & explanation
Correct answer: B
Concept
In a “preceded by / followed by” series-counting question, a term is counted only when BOTH of its immediate neighbours match the stated conditions at the same time. Treat the requirement as a fixed three-term block — (preceding digit)(target)(following digit) — and slide this window across the series, testing every occurrence of the target.
Applying it here
Target = 4, required preceding digit = 2, required following digit = 3, so we look for the consecutive block 2 4 3. Take each 4 together with its immediate neighbours:
Block around the 4 | Preceded by 2? | Followed by 3? | Counts? |
|---|---|---|---|
2 4 3 | Yes | Yes | Yes |
3 4 5 | No | No | No |
7 4 2 | No | No | No |
2 4 5 | Yes | No | No |
2 4 3 | Yes | Yes | Yes |
1 4 7 | No | No | No |
Two of the six 4’s sit inside the block 2 4 3, so exactly two 4’s satisfy both conditions.
Cross-check
Three 4’s have a 2 immediately before them (the blocks 2 4 3, 2 4 5 and 2 4 3). Of these, only the two that are also followed by 3 qualify; the one followed by 5 is dropped. This confirms the count is two.