Because this is a brain teaser, there is no requirement to write 9, so it is ok to use 6 in turn.

Otherwise, there is no solution to this problem, because mathematics can prove that the sum of several odd numbers is even if and only if there is an even number. So it is impossible to add even 30 to three odd numbers.

Generally, people use decimal system to count numbers mathematically, and different systems have solutions. The main systems are binary (B), octal (O), decimal (D), hexadecimal (H) (computer) and hexadecimal (clock), but theoretically any system greater than or equal to 2 is legal.

In decimal system, 30 is what we often call 33, 15. Because the number of digits is different, it means 16 in decimal, so there is15+15 = 30 (16+6550) in decimal. But both of them are undefined decimal systems and will only be used in some non-professional exams.

This is a question that has spread wildly among friends. Many people have already asked.