how to do average in R or xcel : I have a big data set something like this below
 image	        length	       angle

DSC_0001.JPG	13	        22.619865
DSC_0001.JPG	21.470911	27.758541
DSC_0001.JPG	10.198039	11.309933
DSC_0001.JPG	115.97414	60.561512
DSC_0001.JPG	16.27882	79.38035
DSC_0001.JPG	22.803509	15.255118
DSC_0001.JPG	22.825424	28.810793
DSC_0001.JPG	151.0298	1.138177
DSC_0001.JPG	13.038404	85.601295
DSC_0001.JPG	16.124516	7.125016
DSC_0001.JPG	18.027756	3.17983
DSC_0001.JPG	10.198039	11.309933
DSC_0001.JPG	26.832815	26.565052
DSC_0001.JPG	17	61.927513
DSC_0001.JPG	12.165525	80.53768
DSC_0001.JPG	12.206555	55.00798
DSC_0001.JPG	12.727922	45
DSC_0001.JPG	76.02631	63.434948
DSC_0001.JPG	18.248287	9.462322
DSC_0002.JPG	17.492855	59.036243
DSC_0002.JPG	28.160255	6.115504
DSC_0002.JPG	10.049875	84.289406
DSC_0002.JPG	33.970577	42.614056
DSC_0002.JPG	10.440307	16.699244
DSC_0002.JPG	15.231546	66.80141
DSC_0002.JPG	19.723083	30.465546
DSC_0002.JPG	75.802376	53.583622
DSC_0002.JPG	5.8309517	30.963757

I want to do average of "Length" and "angles" for each set(DSC_001 is one set, DSC_002 is another and so on)

I can do it manually in excel but taking huge time when it  around 4000 data point. 

I like to know how I do it in R or in excel in much smarter way?    
