Sneak damage modifier stacking

Question

• Sneak attacks do 6x damage with one-handed weapons
• Sneak attacks with daggers do 15x damage (end perk on skill tree)

A dagger is a one handed weapon, so does this mean either;

1. Daggers just deal 15x damage
2. Daggers deal (15x + 6x) 21x damage
3. Daggers are op and deal (15x * 6x) 90x damage

Answer 1

Consider the text of the perk itself:

Sneak attacks with daggers do 15x damage

The perk is concise and to the point: Daggers do 15x damage. Not, "increased by 15%" or the like, just, "do 15x damage".

This holds up in game, where Daggers do 2.5 times the sneak attack damage of other one-handed weapons.