X can complete a job in 12 days. If X and Y work together

X can complete a job in 12 days. If X and Y work together, they can complete the job in 6⅔ days. Y alone can complete the job in

  1. 10 days
  2. 12 days
  3. 15 days
  4. 18 days

Answer

Efficiency of X = (1/12) × 100 = 8.33 %

Combined efficiency of X and Y = (3/20) × 100 % = 15 %

So, efficiency of Y = (15 - 8.33) % = 6.66 %

Therefore, number of days required by Y = 100 / 6.66 = 15 days

The correct option is C.