A is twice as good a workman as B and they took 7 days together to do the work B alone can do it in.	Pick one:	A) 23 days	B) 25 days	C) 21 days	D) 26 days	E) 29 days

Rationale: "c 21 days wc = 2 : 1 2 x + x = 1 / 7 x = 1 / 21 = > 21 days"
Answer: C