Brain Game: A Single or a Double?

This is a sequence of two-digit numbers, and the last number is missing. Can you identify the final number and tell what these two-digit numbers (but no others) have in common? Good luck!

10, 11, 12, 13, 15, 20, 30, 40, ?

Here is the ANSWER.

THE ANSWER:

50.

And why? These are the only two-digit numbers whose names don't incorporate the spelling of a single digit - zero, one, two, three, four, five, six, seven, eight, or nine.

Of all two-digit numbers, only ten, eleven, twelve, thirteen, fifteen, twenty, thirty, forty and fifty do NOT incorporate the spelling of a single-digit number.

All other two-digit numbers (ex: sixteen, twenty-nine, fifty-eight, sixty, eighty-one, ninety-four) contain the spelling of at least one single digit.

Thanks for playing!