A is half good a work man as B and together they finish a job in 10 days. In how many days working alone B finish the job?
Answers:  A) 23  B) 22  C) 15  D) 36  E) 48

Rationale: "c 15 wc = 1 : 2 2 x + x = 1 / 10 = > x = 1 / 30 2 x = 1 / 15 = > 15 days"
Answer: C