H18J, J17G, L16D, N15A, ______, R39U

Step 1: Look at the letters before the numbers

H → J → L → N → ? → R
The letters are increasing by +2 alphabets each time.
So after N, we get P.

Step 2: Look at the numbers

18 → 17 → 16 → 15 → ? → 39
At first, the numbers are decreasing by 1 each step.
So after 15, the missing number should be 14.
(The jump to 39 happens later as a twist in the sequence).

Step 3: Look at the letters after the numbers

J → G → D → A → ? → U
Here, the letters are decreasing by −3 alphabets each time.
J (10th) → G (7th) → D (4th) → A (1st) → ?
So after A, we move −3 → X (since alphabet cycle continues backwards).

Final Answer:

The missing term is P14X 

This sequence is a mix of three mini-patterns:

The first letter goes forward by 2 alphabets,

The number goes down by 1,

The last letter goes backward by 3 alphabets.

So, filling in the gap gives us: P14X.