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 select columns but you can also deselect them.
# Select the columns to keep.
df %>%
filter(Diet == 3) %>%
mutate(growth = weight/(Time + 1)) %>%
select(weight, Time, Chick, growth)
# Select the columns to remove.
df %>%
filter(Diet == 3) %>%
mutate(growth = weight/(Time + 1)) %>%
select(-Diet, -weight)