What is the mossing number in the sequence, 12356789q0qqq21315161820

Answer 1

1,2,3,5,6,7,8,9,10,11,12,13,15,16,18,20

consecutive integers with 4, 14, 17 and 19 missing

q=1 = the ten's digit of 10, 11, and 12 as well as the ones' digit for 11