When you enroll as a TSA in Ibüümerang, you get a starting amount of Büümerangs to work with:

Standby = 5
Coach Class = 10
Business Class = 25
First Class = 50

The amounts shown are the TOTAL number of Büümerangs that you will have at each level ... they are NOT additive.  

So when you go from Standby to Coach, you get 5 additional Büümerangs so that you have a TOTAL of 10.   When you from Coach to Business Class, you get 15 additional Büümerangs so that you have a TOTAL of 25.   When you go from Business Class to First Class, you get an additional 25 Büümerangs so that you have a total of 50.