This gives the exact p-value from the known distribution
x_obs <- 50
n <- 75
p0 <- .5
binom.test(x_obs, n, p0, alternative = ("greater"))##
## Exact binomial test
##
## data: x_obs and n
## number of successes = 50, number of trials = 75, p-value =
## 0.002614
## alternative hypothesis: true probability of success is greater than 0.5
## 95 percent confidence interval:
## 0.5665713 1.0000000
## sample estimates:
## probability of success
## 0.6666667