Relative frequencies / proportions with dplyr

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Suppose I want to calculate the proportion of different values within each group. For example, using the mtcars data, how do I calculate the relative frequency of number of gears by am (automatic/manual) in one go with dplyr?

library(dplyr)
data(mtcars)
mtcars <- tbl_df(mtcars)

# count frequency
mtcars %>%
  group_by(am, gear) %>%
  summarise(n = n())

# am gear  n
#  0    3 15 
#  0    4  4 
#  1    4  8  
#  1    5  5

What I would like to achieve:

am gear  n rel.freq
 0    3 15      0.7894737
 0    4  4      0.2105263
 1    4  8      0.6153846
 1    5  5      0.3846154

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