Digits of a Page Group

Difficulty: Medium · Topics: Discrete Mathematics, Sets, Word Problems

The pages of a book numbered with consecutive numbers are split into disjoint groups of five, and next to each group the set of the digits used by the numbers in that group is written:

\1,2,3,4,5\→ R 1=\1,2,3,4,5\

\6,7,8,9,10\→ R 2=\0,1,6,7,8,9\

\11,12,13,14,15\→ R 3=\1,2,3,4,5\

…

How many elements does the set R 18cap R 20 have?