
Questions on Combinations and Permutations gives the stories and requires you to provide the solutions. Comparison and studying the reverse gives better understanding of combinations and Permutations. Read on. It helps.

Eg A, B, C
1.
Combination: 3C2 = 3 = AB, AC, BC
The order is not important. It does not matter who comes first. The order is interchangeable ie AB = BA, AC = CA, BC = CB
2.
Permutation: 3P2 = 6 = AB, BA, AC, CA, BC, CB
The order is important. It shows who comes first and who is next.
3.
Factorial:
2! = 2P2 = 2 = AB, BA
3! = 3P3 = 6 = ABC, ACB, BAC, BCA, CAB, CBA
The order is important.
(1), (2) and (3) above looks at 1 group of object only. They look simple. Complication comes in when there are 2 or more groups of objects. When this happens, we need to use a mixture of the above 3 rules. The following scenarios are important as they are the trends of GCE 'O' Level examination questions.
4. Multiplication of Permutation and FactorialGroup 1: A, B, C
Group 2: 1
3P2(Group 1) = 6 = AB, AC, BC, BA, CA, CB
2! (both groups) = 2 = G1G2 or G2G1
3P2(Group 1) x 2!(both groups) = 6 x 2 = 12
= AB1, 1AB, AC1, 1AC, BC1, 1BC, BA1, 1BA, CA1, 1CA, CB1, 1CB
5. Multiplication of Permutation with Permutation and FactorialGroup 1: A, B, C
Group 2: 1, 2, 3, 4
3P2(Group 1) = 6 = AB, AC, BC, BA, CA, CB
4P2(Group 2) = 12 = 12, 21, 13, 31, 14, 41, 23, 32, 24, 42, 34, 43
2!(Group 1 & Group 2) = 2 = G1G2, G2G1
3P2(Group 1) x 4P2(Group 2) x 2!(Group 1 & Group 2) = 6 x 12 x 2 = 144
AB12 12AB AB21 21AB AB13 31AB AB14 14AB AB41 AB41 AB23 23BA AB32 32AB AB24 24AB
AC12 12AC AC21 21AC AC13 31AC AC14 14AC AC41 AC41 AC23 23AC AC32 32AC AC24 24AC
BC12 12BC BC21 21BC BC13 31BC BC14 14CB BC41 41BC BC23 23BC BC32 32BC BC24 24BC
BA12 12BA BA21 21BA BA13 31BA BA14 14AB BA41 41BA BA23 23BA BA32 32BA BA24 24BA
CA12 12CA CA21 21CA CA13 31CA CA14 14CA CA41 41CA CA23 23CA CA32 32CA CA24 24CA
CB12 12CB CB21 21CB CB13 31CB CB14 14CB CB41 41CB CB23 23CB CB32 32CB CB24 24CB
AB42 42AB AB34 34AB AB43 43AB AB31 31AB
AC42 42AC AC34 34AC AC43 43AC AC31 31AC
BC42 42BC BC34 34BC BC43 43BC BC31 31BC
BA42 42BA BA34 34BA BA43 43BA BA31 31BA
CA42 42CA AC34 34AC AC43 43AC AC31 31AC
CB42 42CB CB34 34CB CB43 43CB CB31 31CB Phew!
Now that you have mastered the patterns and properties of Combination, Permutation and Factorial and their multiplication. The challenges lies with the identification on what are the groups. Practice makes perfect.
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