A waiter's salary consists of his salary and tips. During one week his tips were 9/4 of his salary. What fraction of his income came from tips ?
Pick one:
A) 4 / 9
B) 5 / 4
C) 9 / 13
D) 5 / 9
E) 6 / 9

Rationale: "income = salary ( s ) + tips = s + s * 9 / 4 = s * 13 / 4 tips = s * 9 / 4 fraction of his income came from tips = ( s * 9 / 4 ) / ( s * 13 / 4 ) = 9 / 13 answer : c"
Answer: C