Class 11

Math

Algebra

Permutations and Combinations

If the permutation of $a,b,c,d,e$ taken all together be written down in alphabetical order as in dictionary and numbered, then the rank of the permutation $debac$ is:

- 90
- 91
- 92
- 93

$a....=4!$

$b....=4!$

$c....=4!$

$da...=3!$

$db...=3!$

$dc...=3!$

$dea..=2!$

$debac=1$

Rank $=3×4!+3×3!+2!+1$

$=72+18+2+1$

$=93$