Mutating multiple columns in a data frame using dplyr

Question

I have the following data frame df:

  v1 v2 v3 v4
1  1  5  7  4
2  2  6 10  3

And I want to obtain the following data frame

Answer 1

I think I found a solution:

df %>%
  mutate(n = df[1:(ncol(df)/2)] * df[(1+ncol(df)/2):(ncol(df))]) %>% head()

The result is valid for any number of variables. It only remains a problem with the name of the new variables. This is the result:

  v1 v2 v3 v4 n.v1 n.v2
1  1  5  7  4    7   20
2  2  6 10  3   20   18