Calculating group means

Hi All, 

Sorry for what I imagine is quite a basic question. I have been trying to do is create latency averages for each state (1-8) for each participant (n=13) in each condition (1-10). I'm not sure what function I would need, or what the most efficient ay of calculating this would be. If you have any help with that I would be very grateful.

structure(list(subject = c(1L, 1L, 1L, 1L, 1L, 1L), conditionNo = c(1L, 
1L, 1L, 1L, 1L, 1L), state = c(5L, 8L, 7L, 8L, 1L, 7L), latency = c(869L, 
864L, 1004L, 801L, 611L, 679L)), .Names = c("subject", "conditionNo", 
"state", "latency"), row.names = 3:8, class = "data.frame")

Thanks again,

Laura