What should come in place of the question mark (?) in the given series based on the English alphabetical order? KRB, MTD, OVF, QXH, ?

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, ?