RST is related to 181920 in the same way as GHI is related to:

Let's analyze the alphabetical positions of the letters in the first group:
* R = $18\text{th}$ letter of the English alphabet
* S = $19\text{th}$ letter of the English alphabet
* T = $20\text{th}$ letter of the English alphabet

Concatenating these position numbers gives: $181920$.

Now apply the same rule to GHI:

* G = $7\text{th}$ letter of the English alphabet
* H = $8\text{th}$ letter of the English alphabet
* I = $9\text{th}$ letter of the English alphabet

Concatenating these position numbers gives: $789$.

Therefore, GHI is related to $789$.