set up

library(tidyverse)
## -- Attaching packages -------------------------------------------------------------------------------- tidyverse 1.3.0 --
## v ggplot2 3.3.2     v purrr   0.3.4
## v tibble  3.0.1     v dplyr   1.0.0
## v tidyr   1.1.0     v stringr 1.4.0
## v readr   1.3.1     v forcats 0.5.0
## -- Conflicts ----------------------------------------------------------------------------------- tidyverse_conflicts() --
## x dplyr::filter() masks stats::filter()
## x dplyr::lag()    masks stats::lag()
murders <-read_csv("murders.csv")
## Warning: Missing column names filled in: 'X1' [1]
## Parsed with column specification:
## cols(
##   X1 = col_double(),
##   state = col_character(),
##   abb = col_character(),
##   region = col_character(),
##   population = col_double(),
##   total = col_double(),
##   murder_per_capita = col_double()
## )

Try Mutate

murder_per_hundred_thou <- mutate(murders, murder_per_hundred_thou  = population / 100000)

auto_specs_new <- mutate(auto_specs, hp_to_weight = horsepower / weight)

write.csv (murder_per_hundred_thou, file = "murder_per_hundred_thou")
murder_per_hundred_thou
## # A tibble: 51 x 8
##       X1 state   abb   region population total murder_per_capi~ murder_per_hund~
##    <dbl> <chr>   <chr> <chr>       <dbl> <dbl>            <dbl>            <dbl>
##  1     1 Alabama AL    South     4779736   135        0.0000282            47.8 
##  2     2 Alaska  AK    West       710231    19        0.0000268             7.10
##  3     3 Arizona AZ    West      6392017   232        0.0000363            63.9 
##  4     4 Arkans~ AR    South     2915918    93        0.0000319            29.2 
##  5     5 Califo~ CA    West     37253956  1257        0.0000337           373.  
##  6     6 Colora~ CO    West      5029196    65        0.0000129            50.3 
##  7     7 Connec~ CT    North~    3574097    97        0.0000271            35.7 
##  8     8 Delawa~ DE    South      897934    38        0.0000423             8.98
##  9     9 Distri~ DC    South      601723    99        0.000165              6.02
## 10    10 Florida FL    South    19687653   669        0.000034            197.  
## # ... with 41 more rows