其它
In this fictional "Old Island", all the numbers contain only odd digits. The order of the counting numbers is as follows:  1, 3, 5, 7, $\cdots $ , 19, 31, 33, $\cdots $  What is the 31st counting number in the island? 
Overseas Competition->Knowledge Point->Number Theory Modules->Odd and Even Numbers
1,3,5,7,9 -\/-\/-\/-\/-\/-\/-\/-\/-\/-⑤  11,13,15,17,19-\/-\/-\/-\/-\/-\/-⑤  31,33,35,37,39-\/-\/-\/-\/-⑤  51,53,55,57,59-\/-\/-\/-\/-⑤  71,73,75,77,79-\/-\/-\/-\/-⑤  91,93,95,97,99-\/-\/-\/-⑤  $$111$$  the 31st number is 111. 