Calculate Mean for each Treatment/Trial Interaction in DF

I am create a new DF that summarizes the mean angle per treatment/ 
trial, of
the original DF (see below).  I have had some success using:
(tapply(df$Angle, INDEX=interaction(df$State,
df$Trial),  FUN=mean));

however, this gives the answer as a list, which
means I would have to split the name to get the categories back.  Does
anyone know a simple way to transform the Original DF into Summary DF?

Thanks in advance,

James


Original DF

df

      Angle Trial    Treatment
1    43.1297     1   C
2    62.3613     1   C
3    88.2767     2   C
4    75.2431     2   C
5    91.3668     3   C
6    61.2800     3   C
7    55.5575     1   U
8    69.4661     1   U
9    67.5512     2   U
10   95.5528     2   U
11   75.8689     3   U
12   66.9070     3   U

Summary DF

sdf

      Angle Trial    Treatment
1    52.7455   1   C
2    ...           2   C

snip

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