Refer to the following letter series and answer the question. (Counting to be…

Concept: In the English alphabet, A, E, I, O and U are vowels and every other letter is a consonant. "Immediately preceded by" refers to the letter standing directly to the left, and "immediately followed by" refers to the letter standing directly to the right.

A letter that satisfies both conditions at once therefore sits at the centre of a three-letter window of the form vowel - consonant - vowel. Counting such letters is the same as counting V-C-V windows, and the first and the last letter of any string can never qualify because one of their two neighbours does not exist.

Application to this series:

1. Number the letters from the left: E(1) B(2) J(3) E(4) R(5) A(6) J(7) N(8) I(9) D(10) U(11) R(12) N(13) L(14) U(15) H(16) L(17) T(18) R(19).

2. Mark the vowels: E(1), E(4), A(6), I(9), U(11) and U(15). The remaining thirteen letters are consonants.

3. Slide a three-letter window from left to right and keep only the windows whose pattern is vowel - consonant - vowel.

4. E(4) R(5) A(6): the consonant R has the vowel E directly to its left and the vowel A directly to its right, so both conditions hold.

5. I(9) D(10) U(11): the consonant D has the vowel I directly to its left and the vowel U directly to its right, so both conditions hold.

6. No other window in the string has the pattern vowel - consonant - vowel, so the required count is 2.

Cross-check by consonant blocks: list the consonants lying between each pair of neighbouring vowels. A block of exactly one consonant is precisely a V-C-V window, so the number of one-letter blocks must equal the answer.

Exactly two blocks have length one, which matches the window scan.

Common traps - letters that meet only one of the two conditions:

• B(2): the vowel E is on its left, but the consonant J is on its right.

• J(7): the vowel A is on its left, but the consonant N is on its right.

• R(12): the vowel U is on its left, but the consonant N is on its right.

• H(16): the vowel U is on its left, but the consonant L is on its right.

• L(14): the vowel U is on its right, but the consonant N is on its left.

Result: 2 consonants of the series - the R between E and A, and the D between I and U - are immediately preceded by a vowel and also immediately followed by a vowel.