Well, just as it is, Id say do something like this:
Note to OP: I haven't had a chance to test this, It may have errors.
int BirthNumber, Month, Day;
printf( "Enter your birth number: ");
scanf ( "%d", &BirthNumber);
Month = BirthNumber/40 + 1;
Day = BirthNumber - 40*(Month - 1);
printf( "\nYou were born on %d/%d.\n\n", Day, Month);
Note for people who don't program: The datatype "int" stands for "integer" this means it will (or should, as I said, I haven't tested it) truncate the remainder, so for example, my BirthNumber would be 94, then 94/40 will return 2 (not 2.35), because BirthNumber is an integer. An important distinction, or Day would always be set equal to zero.
Though I don't know how useful it is, the definition of BirthNumber seems somewhat obscure.
Edit: Okay, I tested it and it works. It displays 14/3 for me, which is correct.