Which of the following alphanumeric combinations can replace the question mark (?) in the given series? Z2, X5, U10, ?, L26

Which number will replace the question mark (?) in the following series? 83, 79, 71, 67, 59, 55, ?

Which of the following letter-number clusters will replace the question mark (?) in the given series to make it logically complete? HLE23, JJH21, LHK19, NFN17, ?