Treasure question

Do treasures add or multiply the effectiveness? As in

The base 100% + Chapter 1 100% + Chapter 2 100% + Chapter 3 100% Totaling to 400%

Or multiplying Base 100% x Chapter 1 100% x Chapter 2 100% x Chapter 3 100% Totaling to 800%

Tried finding this on the wiki/guide and couldn't find it.