dice

n <- 1e7

# Max of 2d20

r1 <- sample(1:20,n,T)
r2 <- sample(1:20,n,T)
max.r1r2 <- apply(as.matrix(cbind(r1,r2)),1,max)

# Reroll 1d20 if < N

N <- 10

r3 <- sample(1:20,n,T)
r3[r3 < N] <- sample(1:20,length(which(r3 < N)),T)

# compare using hist() or similar