A and B finish the job in 15 days.while A,B and C can finish it in 5 days . C alone will finish the job in  Choose the correct option:
A) 4.50 days
B) 3.50 days
C) 6.0 days
D) 7.5 days
E) 5.50 days

Rationale: "explanation : 5 = ( 15 * x ) / ( 15 + x ) 75 + 5 x = 15 x 10 x = 75 x = 7.5 answer : option d"
Answer: D