Those number are all powers of 10
10= 10^1
100= 10^2
1000= 10^3
and so on...
Check the picture below.
now, since we know what "f" is, let us use the CNT triangle with the small COR one.

Hey There!
80+70 is 150
180
-150
is 30 so 30 must be your answer
Hope This Helps!!!
Answer is in the file below
tinyurl.com/wpazsebu