> diet_summ <- ChickWeight %>% 
      group_by(Diet, Chick) %>% 
      summarise(weight_gain = last(weight) - first(weight)) %>% 
      group_by(Diet) %>% 
      summarise(mean_gain = mean(weight_gain))
> diet_summ
# A tibble: 4 × 2
    Diet mean_gain
  <fctr>     <dbl>
1      1     114.9
2      2     174.0
3      3     229.5
4      4     188.3