Each of these numerical groups conclude with the exact same number. Your mission, should you choose to accept it, is to identify the concluding number and explain why it's the same in each case.

17, 9, ?

1, 3, 5, ?

20, 6, 3, 5, ?

HERE is the answer.

THE ANSWER:

In each case, the answer is FOUR. The next number in each sequence is the number of letters used to spelling out the previous number.

SEVENTEEN (9 letters) -> NINE (4 letters) -> FOUR
ONE (3 letters) -> THREE (5 letters) -> FIVE (4 letters) -> FOUR
TWENTY (6 letters) -> SIX (3 letters) -> THREE (5 letters) -> FIVE (4 letters) -> FOUR

They all get stuck on the number four, since its name has four letters.