Ram, who is half as efficient as Krish, will take 21 days to complete a task if he worked alone. If Ram and Krish worked together, how long will they take to complete the task?
Pick one: A) 16 days B) 12 days C) 8 days D) 7 days E) 18 days

Rationale: "number of days taken by ram to complete task = 21 since ram is half as efficient as krish , amount of work done by krish in 1 day = amount of work done by ram in 2 days if total work done by ram in 24 days is 21 w amount of work done by ram in 1 day = w amount of work done by krish in 1 day = 2 w total amount of work done by krish and ram in a day = 3 w total amount of time needed by krish and ram to complete task = 21 w / 3 w = 7 days answer d"
Answer: D