> diet_summ <- ChickWeight %>% 
     group_by(Diet, Chick) %>% 
     summarise(weight_gain = max(weight) - min(weight)) %>% 
     group_by(Diet) %>% 
     summarise(mean_gain = mean(weight_gain))
> diet_summ
Source: local data frame [4 x 2]

    Diet mean_gain
  (fctr)     (dbl)
1      1    116.95
2      2    174.90
3      3    231.10
4      4    188.90