p3 <- (3/20) * (2/19) * (1/18)
p4 <- (17/20) * (3/19) * (2/18) * (1/17) * choose(3, 2)
k <- 3:20
p <- choose(k-1, 2) / (20 * 19 * 3)
sum(p)
## [1] 1
barplot(p)
sum(p * k)
## [1] 15.75
n <- 1:1000
a <- 365 * n * (1 - 1/365)^n
plot(a, type = "l")
a[1]
## [1] 364
(-1 + sqrt(1 + 4 * 365))/2
## [1] 18.61151
n <- 30
set.seed(123)
sample(1:365, size = n, replace = TRUE)
## [1] 179 14 195 306 118 299 229 244 14 153 90 91 256 197 91 348 137 355 328
## [20] 26 7 137 254 211 78 81 43 359 332 143