Consider all 7-letter words containing only the letters . Arranging these words in alphabetical order, we get: . What is the position of the word ?
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I will show a simple method to solve problems like this one. I know that it is a combinatorics problem but we can use the principle of numerical bases to solve it.
Let's attribute a number to each alphabet of the set:
A = 0
B = 1
C = 2
E = 3
G = 4
Now we can convert the "words list" to a numerical list:
A A A A A A A = 0 0 0 0 0 0 0
A A A A A A B = 0 0 0 0 0 0 1
A A A A A A C = 0 0 0 0 0 0 2
A A A A A A E = 0 0 0 0 0 0 3
. . .
C A B B A G E = 2 0 1 1 0 4 3
Notice that these numbers are in base 5 . Now we can convert CABBAGE to decimal:
2 ⋅ 5 6 + 0 ⋅ 5 5 + 1 ⋅ 5 4 + 1 ⋅ 5 3 + 0 ⋅ 5 2 + 4 ⋅ 5 1 + 3 ⋅ 5 0 = 3 1 2 5 0 + 6 2 5 + 1 2 5 + 2 0 + 3 = 3 2 0 2 3
In this list AAAAAAA is in the "0th" position because it corresponds to the number 0. But it is in actually the 1st. So, CABBAGE is in the 3 2 0 2 3 + 1 = 3 2 0 2 4 t h position in this list.