Once again we assume that you've got the dataframe set up appropriately.
library(tidyverse) # This command makes sure that the chickweight dataframe is clean. df <- ChickWeight %>% ungroup() %>% as.data.frame()
You can combine verbs together. Below we're using filter, mutate, select and arange.
df %>% filter(Diet == 3) %>% mutate(growth = weight/(Time + 1)) %>% select(-Diet, -weight) %>% arrange(-growth)