161981

First we wrote out all the possible domino combinations and worked out how many tiles there were (which was 28).
We then added up all the spots from these combinations and got 168.
We took away 125 (the amount in Amy's bag) from 168 to get 43. Then we divided it by 4 to get the mean number of spots on each tile (10 and three quarters!).
We knew they had to be the biggest numbers; we used the 4 largest tiles which gave us a total of 43. (6+6=12; 5+5=10; 5+6=11; 4+6=10) Then 12 + 10 + 11 + 10 = 43.
So the missing tiles were: 6:6, 5:5, 5:6 and 4:6. We think that's the only combination because any other combination of 43 requires more than 4 tiles.