There are 24 "different" cars:
(S,B,M)
(S,B,T)
(S,B,C)
(S,B,P)
(S,W,M)
(S,W,T)
(S,W,C)
(S,W,P)
(S,R,M)
(S,R,T)
(S,R,C)
(S,R,P)
(A,B,M)
(A,B,T)
(A,B,C)
(A,B,P)
(A,W,M)
(A,W,T)
(A,W,C)
(A,W,P)
(A,R,M)
(A,R,T)
(A,R,C)
(A,R,P)
Notice that there are 4 ways to pair the Toyotas with a "different" Mercedes-Benz, "different" Chevrolet or "different" Pinto. So, 4 + 4 + 4 = 12
And there are 4 ways to pair the Mercedes-Benz with a "different" Chevrolet or "different'' Pinto. {We've already paired each with a "different" Toyota.} So 4 + 4 = 8
And since we've already paired the Chevrolet with a "different" Toyota or Mercedes-Benz, we have 4 ways that we can pair them with a "different" Pinto.
So there are 12 + 8 + 4 = 24 "different" pairings
And the total number of possible pairs made by selecting any 2 cars from 24 = C(24,2) = 276
So the probability is 24 / 276 = 2/23.