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)