Respuesta :
a b c d
ab
ac
ad
bc
bc
cd
6 different kinds, assuming cannot have double of same, otherwise:
a b c d
aa
ab
ac
ad
bb
bc
bd
cc
cd
dd
10 different kinds
ab
ac
ad
bc
bc
cd
6 different kinds, assuming cannot have double of same, otherwise:
a b c d
aa
ab
ac
ad
bb
bc
bd
cc
cd
dd
10 different kinds