Q39 / 90 · JEE Main 2014 (09 Apr Online) · Mathematics

The sum of the digits in the unit's place of all the $4$ - digit numbers formed by using the numbers $3, 4, 5$ and $6$ , without repetition is :

A
$18$
B
$36$
Correct answer
C
$108$
D
$432$

Solution

By fixing one digit at unit place we can form toal number without repetition is $3!$

Sum of digits $= 3! . (6 + 5 + 4 + 3)$

$= (6) (18) = 108$ $\{∵ n! = n . (n - 1) . . . 3.2.1\}$

Hence, the correct answer is option: 'C'.

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