A) 640
B) 320
C) 420
D) 510
Correct Answer: D
Solution :
[d] Let x, y, z be the friends and a, b, c denote the case when x is now, we have the following possibilities: (a, b, c) = (1, 2, 3) or (3 3 0) or (2 2 2) [grouping of 6 days of week] Hence, the total number of ways is \[\frac{6!}{1!2!3!}3!+\frac{6!}{3!3!2!}3!+\frac{6!}{(2!2!2!)}3!\]
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