The Funny Book has its pages numbered the following way:
1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5 ... That is, there is one 1, two 2s, three 3s, four 4s, five 5s, and so on.
What is the greatest number of pages that the Funny Book can have if it contains a page that is
numbered “20” but not any that are numbered “21”?