Instead of the 12/18 rule, we use the 15/+3/-3 Rule. Meaning that all Attribute rolls Difficulty Level starts out at 15. If a primary attribute is used, add 3. If Secondary, subtract three.
D’arcon wants to disarm a trap.
The trap was created by a 5th level Rogue.
The DC is 20 (15 + 5, the level of the Rogue making the trap).
The roll uses the Dexterity Attribute. D’acron has a 16 and it’s a Prime attribute for him.
So the roll is 1d20 5(for level)+ 3 (because it’s Prime)
He rolls a 10 and wins the roll (10+2+5+3=20).