adding a sub total in a column

[yoyong]

Hi everyone.

I have a table which looks like the 1st 3 columns below. I wanted to add two more columns.
The fourth column is the total for each age_group and the last the proportion (2 dp) (3rd column/4th column).
Not sure how do do it.

code	age_group	n	total	proportion
0	0-5	12	42	0.29
1	0-5	15	42	0.36
2	0-5	10	42	0.24
3	0-5	5	42	0.12
0	6-10	7	61	0.11
1	6-10	21	61	0.34
2	6-10	15	61	0.25
3	6-10	18	61	0.30
0	11-15	20	57	0.35
1	11-15	25	57	0.44
2	11-15	10	57	0.18
3	11-15	2	57	0.04

Thanks.

[nirgrahamuk]

library(tidyverse)
(madeupdf <- data.frame(c=rep(0:3,2),
                       a = sort(rep(letters[1:2],4)),
                       n = 1:8 *10))

(total_df <- group_by(madeupdf,
                     a) %>% summarise(
                       total=sum(n)
                     ))

(result_df <- left_join(madeupdf,
                        total_df) %>% 
                  mutate(prop = round(n/total,2)))