#1
  ch = c("0","21","12","16")
  #2
  y = as.numeric(ch)
  y = sort(y)
  y
  #3
  y2 = seq(0,24,by=8)
  y2
  #4
  sum((y < y2) == (y<=y2))
  #5
  x = seq(from=1,by=2,length=30)
  x
  #6
  rep(c(T,F),times=5)
  #7 
  sum(x[x>10 & x<20])
  #8
  xmat = matrix(x,6,5,byrow=T)
  #9
  z = list(
  sumr=apply(xmat,1,sum),
  meanc=apply(xmat,2,mean),
  varc=apply(xmat,2,var))
  z