library(dplyr)
dat<-data.frame(S=c(rep(1,8),rep(2,6)),
                    A=c(rep("L",4),rep("R",4),rep("L",4),rep("R",2)),
                        B=c(rep(c("pic","pic","word","word"),3),"pic","pic"),
                            Y=c(1234,1883,732,273,1843,1362,321,234,2983,1283,321,532,2492,2182))
dat %>% group_by(S,A,B) %>% summarise(mean(Y))->answer