I Have a data frame with gender, class and age. I need a result table that gives mean of age by gender and class, difference of mean with observation

Question

My data frame is something like this. dat1 <- data.frame(age = c(21, 20, 17,20, 12, 13, 15,16), Gender = factor(c(2, 2, 1, 2, 1, 2, 1, 1), labels = c("Female&