disc <- data.frame(
    promote = c(rep("promoted", 35), rep("not_promoted", 13)), 
    sex = c(rep("male", 21), rep("female", 14),
    rep("male", 3), rep("female", 10)))

disc %>%
    group_by(sex) %>%
    summarize(promoted_prop = mean(promote == "promoted"))

# A tibble: 2 × 2
     sex promoted_prop
  <fctr>         <dbl>
1 female     0.5833333
2   male     0.8750000