P&C

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karthikeyan.srinivasan: Newbie | Next Rank: 10 Posts; Posts: 2; Joined: Sat Mar 05, 2011 8:39 pm

by karthikeyan.srinivasan » Fri Mar 11, 2011 7:47 am

Q: There are 21 identical Ts and 19 identical Es. In howmany ways can these items can be placed in a Shelf such that no two Es are together?

A: 1540

can anybody explain?

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Source: Beat The GMAT — Problem Solving | Fri Mar 11, 2011 7:47 am

HSPA: Legendary Member; Posts: 1101; Joined: Fri Jan 28, 2011 7:26 amFollowed by: 13 members; Thanked: 47 times; GMAT Score:640

by HSPA » Fri Mar 11, 2011 9:35 am

How did you get 22C19 .... I am getting 23C19

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HSPA: Legendary Member; Posts: 1101; Joined: Fri Jan 28, 2011 7:26 amFollowed by: 13 members; Thanked: 47 times; GMAT Score:640

by HSPA » Fri Mar 11, 2011 9:41 am

instead of 21 and 19 consider 3 M and 2 F and __ is the spaces

__ M __ M__ M __

So here you see we can fill 2F in 4 spaces ... so it is in 4C2 ways = (3+1)C2 = (21+1)C19 = 1540

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