Selecting and modifying rows in a dataframe based on criteria

You can do :

temp <- X[substring(X$V1, 2, 2) == 5, ]

So answer to question a) is: temp$V1

and question b) is: temp$V2

For question c), we can use substring to paste 1st to 3rd letter of V2, insert "X" in between and the remaining string to create new column as V3.

temp$V3 <- paste0(substring(temp$V2, 1, 3), 'X', substring(temp$V2,  4))