total possible numbers with {a,b,c} = 35
possible letter with {a,b} = 25 (which includes aaaaa, bbbbb)
possible letter with {a,c} = 25 (which includes aaaaa, ccccc)
possible letter with {b,c} = 25 (which includes bbbbb, ccccc)
Required = all possible - possible only with ab, ac, bc
= 35 - {25 + 25 + 25 - 3} ; (3 bcz aaaaa, bbbbb, ccccc are occuring 2 times)
= 243 - (96-3)
= 150