Am not trashing MGMAT's guides...in fact, I'm a fan. But, I think the combinatorics section could've been written a bit more clearer, coz I remember being able to solve these problems using easier techniques in the past.
I googled, and chanced upon this resource...that made my day. http://www.mathsisfun.com/combinatorics ... tions.html
Clear, Crisp & Concise formulas for the 4 categories of problems:
[n is the size of the pool, r is the # of objects we're selecting from the pool]
1) ORDER MATTERS = Y, REPETITIONS = allowed
Formula: n raised to the power r
2) ORDER MATTERS = Y, REPETITIONS = not allowed (nPr)
Formula: n! / (n-r)!
3) ORDER MATTERS = N, REPETITIONS = allowed
Formula: (n+r-1)!/( (r!) (n-1)!)
4) ORDER MATTERS = N, REPETITIONS = not allowed (nCr)
Formula: n! / ( (r!) (n-r)! )
Thought I'd share this for others struggling to remember what you learned years ago...