Jan 062012
Combinations are a probability concept that should be understood well before test day.
•A combination of a group of objects is a selection of objects in which the order is not relevant.
–Let n be the total number of objects and r the number of available slots.
Then the total number of combinations where a set of r elements is selected from among a set of n elements is denoted nCr.
nCr = n!/r!x(n-r)!
–Example: Find the number of committees of three people that can be formed from five people.
5C3 = 5!/3!x(5-3)! = 5x4x3x2x1/3x2x1x2x1 = 5×4/2×1 = 10