DAY 409

WHAT'S NEXT?
Credit: Will Shortz
NPR Weekend Edition Sunday Puzzle

What is the next number in this series?

2, 4, 6, 9, 11, 15, 20, 40, 51, 55, 60, 90, ???

Answer:

101 ............... Each number can be expressed in Roman Numerals by two digits:

II, IV, VI, IX, XI, XV, XX, XL, LI, LX, XC, CI