move <- function(i, j, r)
{
    x <- c(i, j)
    A[x, ] <<- A[x, ] + c(-r, r) %o% A[i, ]
}

A <- cbind(rep(1, 3), c(0.25, 0.33, 0.50))
B <- rbind(c(0, 1, 0.1), c(1, 2, 0.2), c(0, 2, 0.3))
k <- 2

apply(B, 1, function(v) move(v[1] + 1, v[2] + 1, v[3])) -> .
cat(A[k + 1, 2] / A[k + 1, 1], "\n", sep = "")