If n is an integer, f(n) = f(n-1) - n and f(4)=20. What is the value of f(6)?  Choose the correct choice from the following options:
A) 9 B) 0 C) 1 D) 2 E) 4

Rationale: "since f ( n ) = f ( n - 1 ) - n then : f ( 6 ) = f ( 5 ) - 6 and f ( 5 ) = f ( 4 ) - 5 . as given that f ( 4 ) = 20 then f ( 5 ) = 20 - 5 = 15 - - > substitute the value of f ( 5 ) back into the first equation : f ( 6 ) = f ( 5 ) - 6 = 15 - 6 = 9 . answer : a . questions on funtions to practice :"
Answer: A