Search found 2 matches
One good one is that xC1 = xC(x-1) = x
This can save time and scribblings in combinations questions.
- by deirdre
Thu May 14, 2009 7:21 am- Forum: GMAT Math
- Topic: What's YOUR favorite GMAT Quantitative shortcut?
- Replies: 21
- Views: 6374
An essentially similar way is to consider all the possible 4-digit combinations then to subtract those which begin with zero.
Total: 4C2 x (1C1)^2 x (9C1)^2 = 6x1x1x9x9 = 486
Beginning with 0: 1C1 x 3C2 x (1C1)^2 x 9C1 = 1x3x1x1x9 = 27
Ans: 486-27 = 459
- by deirdre
Thu May 14, 2009 7:17 am- Forum: Problem Solving
- Topic: '3' exactly twice
- Replies: 22
- Views: 4558