28.7

n = 20
estd = 50
df = NULL
df <- rbind(df, data.frame(Type="Krakow", Strength=rnorm (n, mean=175.0, sd=estd)))
df <- rbind(df, data.frame(Type="Bunnell", Strength=rnorm (n, mean=175.0, sd=estd)))
df <- rbind(df, data.frame(Type="Hybrid", Strength=rnorm (n, mean=275.0, sd=estd)))
df <- rbind(df, data.frame(Type="Control", Strength=rnorm (n, mean=150.0, sd=estd)))
df$Type <- as.factor(df$Type)
summary <- cbind(
    aggregate(df$Strength, list(df$Type), mean),
    aggregate(df$Strength, list(df$Type), sd)[,2]
)
colnames(summary) <- c("Type", "Mean", "SD")