What comes next? G7, T20, H8, S19, ?

Correct — D.
Rule: two series alternate, and each letter carries its own alphabet position.

Odd places: G7, H8, letter +1 and number +1.
Even places: T20, S19, letter −1 and number −1.

The fifth term sits in an odd place, so it continues G7, H8.
After H comes I, letter 9 → I9, option (d).

Why the others are wrong

• (a)
  Q17 — Q17 is a true letter–position pair, but Q comes after R in the falling T, S series. It would be the eighth term, not the fifth.
• (b)
  J10 — J10 skips a letter. The rising series runs G, H, I, J, so J10 is the seventh term, after I9.
• (c)
  R18 — R18 continues T20, S19, the falling series. That series fills the even places, so R18 is the sixth term, one step too late.

An alternating series is two series woven together. Split it by position, terms 1, 3, 5 in one line and terms 2, 4, 6 in the other, and solve each line on its own.

Here every number is simply the alphabet position of its letter (G = 7, T = 20), so all four options are valid pairs. What decides the answer is which line the fifth term belongs to, and that comes from its place, not its value.

Key facts

• Alphabet positions used here: G = 7, H = 8, I = 9, S = 19, T = 20.
• Odd-place terms rise: G7, H8, I9, J10.
• Even-place terms fall: T20, S19, R18, Q17.

Common traps

• Continuing the last term shown (S19 → R18) instead of the series the fifth place belongs to
• Checking only that the number matches its letter, which all four options do

No directly related past PYQ was found.