Refer to the following letter series and answer the question that follows.…

Concept: In a letter-series test, "immediately preceded by" and "immediately followed by" refer to the two neighbours that touch a letter, so a condition of this kind fixes a three-letter window with the counted letter in the middle. The window demanded here is vowel - consonant - consonant: a vowel on the left, the counted consonant in the middle, and another consonant on the right. A, E, I, O and U are the vowels and every other letter, Y included, is treated as a consonant; a letter standing at either end of the series can never qualify, because one of its two neighbours does not exist.

Application: Number the given series from the left - 1 M, 2 R, 3 E, 4 S, 5 I, 6 C, 7 G, 8 T, 9 B, 10 Y, 11 Z, 12 O, 13 U, 14 K, 15 X, 16 W, 17 A, 18 Q, 19 D, 20 F, 21 H. Every qualifying window has to open at a vowel, so the quickest scan is to take each vowel in turn and read the two letters that follow it.

1. E at position 3: the next two letters are S and I. S is a consonant, but I is a vowel, so the window E-S-I is vowel-consonant-vowel and S is not counted.

2. I at position 5: the next two letters are C and G, both consonants. The window I-C-G is vowel-consonant-consonant, so C is counted.

3. O at position 12: the letter that follows it is U, which is itself a vowel, so the window fails at its very first step.

4. U at position 13: the next two letters are K and X, both consonants. The window U-K-X is vowel-consonant-consonant, so K is counted.

5. A at position 17: the next two letters are Q and D, both consonants. The window A-Q-D is vowel-consonant-consonant, so Q is counted.

The letters that survive the scan are C, K and Q, so the number of such consonants is 3.

Cross-check: run the same test from the other side, over the consonants rather than the vowels. Four consonants in the whole series have a vowel sitting immediately on their left, and each of them is then judged by the letter on its right.

• S, which follows E: the letter after it is I, a vowel, so S drops out.

• C, which follows I: the letter after it is G, a consonant, so C survives.

• K, which follows U: the letter after it is X, a consonant, so K survives.

• Q, which follows A: the letter after it is D, a consonant, so Q survives.

Both routes leave the same three letters, C, K and Q, which confirms the tally of 3. Note also that Y at position 10 is treated as a consonant in this test; the neighbours of Y are B and Z, which are consonants under either convention, so the status of Y does not change the count.