One night 20 percent of the female officers on a police force were on duty. If 100 officers were on duty that night and half of these were female officers, how many female officers were on the police force?
Choose the correct choice from the following answers:  A) 90  B) 180  C) 250  D) 500  E) 1,000

Rationale: "let total number of female officers in the police force = f total number of officers on duty on that night = 100 number of female officers on duty on that night = 100 / 2 = 50 ( 20 / 100 ) * f = 50 = > f = 250 answer c"
Answer: C