Thanks Eighteen telephones have just been received at a auth

Thanks!

Eighteen telephones have just been received at a authorized service center. Six of these telephones are cellular,1, 2,...,18 to establish the order in which they will be serviced.(Round your answers to four decimal places.)

Solution

a)

ways of choosing 6 phones = C(18,6) = 18564
ways of choosing 6 cordless phones among first 12 to be serviced = C(12,6) = 924

Prob = 924/18564 = 0.0498

b)
ways of choosing last 6 phones = C(18,6) = 18564
ways of choosing 2 of 3 types = C(3,2) = 3
ways of choosing last 6 phones from these 2 types = C(12,6) = 924
ways of choosing 0 from either of these 2 types = 2C(6,6)C(6,0) = 2
Prob = 3×(924-2)/18564 = 461/3094 = 0.1490
c)
ways of choosing first 6 phones = C(18,6) = 18564
ways of choosing 2 cell, 2 cordless, 2 corded = C(6,2)*C(6,2)*C(6,2) = 3375

Prob = 3375/18564 = 0.1818