Study the given pattern carefully and select the number that can replace the question mark (?) in it. (6, 4, 6) (18, 9, 13) (8, 6, ?) ( NOTE : Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. E.g. 13-Operations on 13 such as adding/subtracting/multiplying etc. to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed)