library(data.table)
heart_RDT=fread("heart.csv",header=TRUE)
ff<-heart_R%>%filter(chol > 280 & sex==1)
#Mean age of men with chol > 280
mean(ff[,1])
#Mean age computed from a data.table
dd<-heart_RDT[chol > 280 & sex==1,.(mm=mean(age))]
dd
55.05