tiro <- c("CENTRO","ERRORE")
prove <- 100000
num <- 0; den <- 0
for (i in 1:prove)
{
    A <- sample(tiro,size=1,prob=c(3/4,1/4))
    B <- sample(tiro,size=1,prob=c(1/2,1/2))
    C <- sample(tiro,size=1,prob=c(2/5,3/5))
    out <- c(A,B,C)
    if (sum(out=="CENTRO")==1) {
       den <- den + 1
       if (A=="CENTRO") num <- num+1
    }
}

cat(num,den,num/den,"\n")