What is the next letter in the series A, C, F, J, O, ...?
SHORT ANSWER
U. Using positions A = 1, C = 3, F = 6, J = 10, O = 15, the gaps are 2, 3, 4, 5, so the next gap is 6 and the next position is 21, which is U.
Explain it simply
Convert letters to their alphabet positions and the letter series becomes a number series. Memorising a few anchors speeds this up: E = 5, J = 10, O = 15, T = 20, Y = 25. The memory hook "EJOTY" helps you jump to any letter quickly without counting on your fingers.
Example
Question: A, C, F, J, O, ?
- Positions: A = 1, C = 3, F = 6, J = 10, O = 15.
- Gaps: +2, +3, +4, +5.
- Next gap: +6, giving 21.
- Position 21 = U (T is 20, so U is 21).
These positions are also triangular numbers (1, 3, 6, 10, 15, 21).
If the count goes past 26, wrap round: position 27 is A again.
js
const pos = [1, 3, 6, 10, 15, 21];
console.log(pos.map(p => String.fromCharCode(64 + p)).join(",")); // Output: A,C,F,J,O,UWhat to say in the interview
"I convert to positions, 1, 3, 6, 10, 15. The gaps go up by one each time, so the next gap is 6 and the position is 21, which is U."
Common mistakes
- Miscounting alphabet positions; use the EJOTY anchors.
- Forgetting to wrap round after Z in longer series.