Using as.numeric() works for me.
X <- c("42.75", "N/A", "48.91", "26.92", "29.95", "28.30")
Xnum <- as.numeric(X)
Warning message:
NAs introduced by coercion
Xnum
[1] 42.75 NA 48.91 26.92 29.95 28.30
class(Xnum)
[1] "numeric"
The warning is displayed because "N/A" cannot be converted to a number.
I chose to store the result in a new variable named Xnum but you could overwrite the original variable X with the result of as.numeric(X).