3.1, 3.1. 33.11, 33.111 what are the next two numbers

Question

how do you work finding the next two numbers after

3.1, 3.11, 33.11, 33.111

Answer 1

3.1, 3.11, 33.11, 33.111, 333.111, 333.1111.

The numbers of 1's and 3's increase stepwise through the serie, with 1's increasing before 3's.

3.11 is 3.1 with an added 1; 33.11 has an added 3 while the 1's stay the same; then the 1's increase again to 33.111; then the 3's catch up: 333.111; now it's the turn of the 1's: 333.1111.