MA
MT
MH
AM
AT
AH
TM
TA
TH
HM
HA
HT
first time you can choose 4
second time you can choose 3
total: 4*3=12 but, there are 2 (=2!) ways two letters can be arranged, so you have to divide by that. so (3*4)/2= 6
"from n choose r":
[input]ncr(4,2)[/input]
