loops through sequential subsets of a binary vector and randomly changes 1s to 0 until the mean of each subset is below 0.40.

b <- c(1,1,1,1,1,1,1,0,0,0,0,0,0,0)

while (mean(b) > 0.40) {
  b[sample(which(b==1), 1)] <- 0
}