Letter and Symbol Series Questions and Answers

Answer: A

Correct answer: I10

Each term pairs a letter with a number, and both advance by two: the letters run A, C, E, G at two places apart, and the numbers run 2, 4, 6, 8. The next letter is I and the next number 10, giving I10. Compound terms are best read as two independent series that happen to be printed together. I8 keeps the letter right but repeats the number, which is the commonest slip. Because both strands advance by two, the term at any position can be written down directly without working through the intervening terms.

Answer: B

Correct answer: J15

The letters advance two places at a time - B, D, F, H - while the numbers advance three at a time: 3, 6, 9, 12. The next term therefore pairs J with 15, giving J15. The two strands move at different rates, which is what distinguishes this from the simpler compound series where both advance by the same amount. K15 takes a three-step in the letters as well and is the natural error.

Answer: C

Correct answer: R10

The letters descend two places at a time from the end of the alphabet - Z(26), X(24), V(22), T(20) - while the numbers climb by two. The next letter is R at position 18 and the next number is 10, giving R10. Letters falling while numbers rise is a deliberate distraction; separating the strands makes both trivial. S at 19 would be a single backward step. The letters and numbers move in opposite directions but at the same rate, so the sum of the letter's position and the number stays constant at 28 throughout.

Answer: D

Correct answer: E25

The letters simply advance one place at a time. The numbers are 1, 4, 9, 16 - the squares of 1, 2, 3 and 4 - so the fifth term takes the square of 5. The answer is E25. Recognising a square sequence is quicker than tracking the differences, which here are the odd numbers 3, 5 and 7. E20 follows from assuming the numbers grow by a constant amount. The differences between the squares are 3, 5, 7 and 9, the odd numbers, which is the standard way to recognise a square series when the squares themselves are not obvious.

Answer: A

Correct answer: O125

Two rules run together. The letters advance three places at a time: C, F, I, L, so the next is O. The numbers are 1, 8, 27, 64 - the cubes of 1, 2, 3 and 4 - so the next is the cube of 5, which is 125. The term is O125. O100 mistakes the cubes for squares, which agree at the first two terms and diverge sharply afterwards. Cubes and squares agree at 1 and diverge at once afterwards, so checking the second term is enough to tell the two rules apart.

Answer: B

Correct answer: Q80

The letters advance one place at a time - M, N, O, P - so the next is Q. The numbers double at each step: 5, 10, 20, 40, so the next is 80. The term is Q80. Q60 comes from reading the numbers as growing by a constant 20, which fits the last step but not the earlier ones; checking the first pair as well as the last is what rules it out. A doubling series reaches large numbers quickly, so the arithmetic is worth doing in stages rather than in one step as the terms grow.

Answer: C

Correct answer: E60

The letters climb one place at a time while the numbers fall by ten: 100, 90, 80, 70. The next term pairs E with 60, giving E60. Compound terms in which one strand rises and the other falls are common and are handled the same way as any other - read the two independently. E50 takes two steps of ten at once, which is the error a hurried count produces. The numbers falling by ten while the letters climb is a deliberate distraction; nothing links the two strands, and each is trivial on its own.

Answer: D

Correct answer: O15

Each term pairs a letter with its own position in the alphabet: K is the 11th letter, L the 12th, M the 13th, N the 14th. The next letter is O, the 15th, so the term is O15. Once the relationship between the two halves is seen there is nothing to calculate. O16 keeps the right letter but advances the number twice, which happens when the pairing is missed and the number is treated as an independent series.

Answer: A

Correct answer: MN

Each pair is two consecutive letters, and the pairs themselves start three places apart: A, D, G, J. The next pair begins at M and so is MN. Two things are moving at once - the internal structure of each pair and the step between pairs - and both have to be checked. LM starts one place too early, which is what following the letters continuously rather than in threes produces. The pairs consume two letters each and advance by three, so one letter is skipped between every pair - C, F, I and L in turn.

Answer: B

Correct answer: EV

The first letter of each pair climbs from the start of the alphabet - A, B, C, D - while the second falls from its end: Z, Y, X, W. The next pair is EV. Each pair's two letters sum to 27 in position, which is a quick way to check any term: E is 5, V is 22. EW keeps the correct first letter but repeats the previous second letter, the slip that comes from tracking only one half. Because the two letters of each pair always sum to 27, any pair can be checked in isolation without reference to the rest of the series.