devarshi-dt-logo

Question:

8-digit numbers are formed using the digits 1, 1, 2, 2, 2, 3, 4, 4. The number of such numbers in which the odd digits do not occupy odd places, is :

60

48

160

120

Solution:

Odd = 1, 1, 3
Even = 2, 2, 2, 4, 4
(2,2,2,4)+(2,2,4,4)⇒4!3!×4!2!+4!2!2!×4!2!⇒4×12+6×12⇒48+72=120