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)