[R] Summarize by two or more attributes

[LCOG1]

Okay everyone heres a likely softball for someone.

Consider the following data frame:

#Create data
x<-rep(c(1,15),10)
y<-rnorm(20)
z<-c(rep("auto",10),rep("bus",10))
a<-rep(c(1,1,2,2,3,3,4,4,5,5),2)
#Create Data frame
Df<-data.frame(Source=x,Rate=y,Bin=a,Type=z)


I want to create a new column the equals the sum of the Rates for each type
(1,15) by Bin.  

A related question:  I have been using R for a while now and usually
manipulate my data in data frames but i know lists are better for R so
perhaps the above should be done using lists.  Feel free to offer
suggestions coming from that angle.  

Thanks guys

JR-



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