Letter codes & sequences

Letters as numbers: A=1, B=2, C=3, … Z=26. Every answer gives the rule first, then the arithmetic.

Timed drill

(b) J. Gap +2 each time: B, D, F, H, J.
(c) V. Gap -1 each time: Z, Y, X, W, V.
(b) O. Gap +3 each time: C(3), F(6), I(9), L(12), O(15).
(b) P. Gap +3 each time: A, D, G, J, M, P.
(b) M. Gap -3 each time: Y(25), V(22), S(19), P(16), M(13).
(b) U. Gaps grow +2, +3, +4, +5, next +6. O(15) + 6 = 21 = U.
(a) EPH. Shift +1: D+1=E, O+1=P, G+1=H.
(b) +3. B+3=E, A+3=D, T+3=W.
(a) A. Gap -3: M(13), J(10), G(7), D(4), A(1).
(b) X. Two interleaved sequences: odd positions A, B, C (+1); even positions Z, Y, ? (-1). Next even = X.
(c) Q. Gap +1: P, Q, R, S.
(c) T. Gap +4: D(4), H(8), L(12), P(16), T(20).
(b) CAT. The shift from DOG to GRJ is +3. Reverse (-3) on FDW: F-3=C, D-3=A, W-3=T.
(a) IJ. Each pair steps +2 from the previous: AB, CD, EF, GH, IJ.
(b) T. Gaps grow +3, +4, +5, next +6. N(14) + 6 = 20 = T.
(a) EV. First letters A, B, C, D, E (+1); second letters Z, Y, X, W, V (-1).
(a) H. Gap -3: T(20), Q(17), N(14), K(11), H(8).
(a) WLT. Mirror: 27 - position. D(4) -> 23 = W; O(15) -> 12 = L; G(7) -> 20 = T.
(c) V. Gaps +1, +2, +3, +4, +5, next +6. P(16) + 6 = 22 = V.
(b) Q. Gap +2: M, O, Q, S, U.

(a) F. Gaps -2, -3, -4, -5, -6. L(12) - 6 = 6 = F.
(b) OEOV. Applying shifts -1, -2, -3, -4: P-1=O, G-2=E, R-3=O, Z-4=V.
(a) IR. First letters A, C, E, G, I (+2); second letters Z, X, V, T, R (-2).
(b) W. Gaps +1, +2, +3, +4, +5, next +6. Q(17) + 6 = 23 = W.
(c) FOOD. Mirror: 27 - position. U(21) -> 6 = F; L(12) -> 15 = O; L -> O; W(23) -> 4 = D.
(a) Y. Gaps +3, +5, +7, next +9 (odd numbers). P(16) + 9 = 25 = Y.
(a) X. Gaps +1, +2, +3, +4, +5, next +6. R(18) + 6 = 24 = X.
(a) Friday. Shift is +1, so decode by -1: G-1=F, S-1=R, J-1=I, E-1=D, B-1=A, Z-1=Y. FRIDAY.