divide columns using pattern in colnames

Assuming I have the following simplified table which has dynamic columns a_x (where x is an index e.g 0, 1, 2, 3, 4...) and b_x respectively. The number of a columns is always equal to the number of b columns but the total number of columns can be dynamic (not always 3 a and 3 b). To make it clearer the following example depicts the structure of my data:

> d <- read.table(text = "10 20 25 0.3 0.23 0.34 
                          40 20 30 0.25 0.4 0.45")
> names(d) <- c("a_0", "a_1", "a_2", "b_0", "b_1", "b_2")
> d
   a_0 a_1 a_2  b_0  b_1  b_2
1  10  20  25   0.30 0.23 0.34
2  40  20  30   0.25 0.40 0.45

I would like to divide a columns with the corresponding b columns and save the results in new c columns. In order to do the divisions I use the transform() function (with hard-coded colnames) like this:

transform(d, c_0 = as.numeric(as.character(a_0)) / as.numeric(as.character(b_0)))

How can I do this step automatically using (probably) a pattern in colnames given the fact the number of columns of my input data is not always the same.

Any help would be appreciated